Guide to the OSHA excavations standard.

A Guide to the
OSHA Excavations Standard
Bobby R. Davis
Series Editor
Division of Occupational Safety and Health
N. C. Department of Labor
1101 Mail Service Center
Raleigh, NC 27699- 1101
Cherie K. Berry
Commissioner of Labor
N. C. Department of Labor
Occupational Safety and Health Program
Cherie K. Berry
Commissioner of Labor
OSHA State Plan Designee
Allen McNeely
Deputy Commissioner for Safety and Health
Kevin Beauregard
Assistant Deputy Commissioner for Safety and Health
Acknowledgments
This edition of A Guide to the OSHA Excavation Standard has been updated to include material as
prepared by the Construction Education and Research Institute, Department of Civil Engineering at
N. C. State University ( principal investigators were Paul P. McCain and David W. Johnston); U. S.
Department of Labor, Occupational Safety and Health Administration; U. S. Department of Health and
Human Services ( NIOSH); and OSHNC personnel. This guide is intended to be consistent with exist-ing
state and federal OSHA standards. Therefore, if the reader considers a statement to be inconsis-tent
with a standard, the OSHA standard should be followed.
To obtain additional copies of this book, or if you have questions about N. C. occupational safety and
health standards or rules, please contact:
N. C. Department of Labor
Bureau of Education, Training and Technical Assistance
1101 Mail Service Center
Raleigh, NC 27699- 1101
Phone: ( 919) 807- 2875 or 1- 800- NC- LABOR ( 1- 800- 625- 2267)
____________________
Additional sources of information are listed on the inside back cover of this book.
____________________
The projected cost of the OSHNC program for federal fiscal year 2000– 2001 is $ 14,152,395. Federal funding provides approximately 30 percent
($ 4,528,766) of this total.
Printed 02/ 02, 1M
Contents
Part Page
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1iiv
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1vii
1 Common Soil Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1ii1
2 Soil Types and Pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . ii18
3 Methods of Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii14
4 Installation and Removal of Protective Systems . . . . . . . . . . . . . . ii18
5 Residential Contractors and the Excavations Standard . . . . . . . . . ii29
6 Worker Training and Jobsite Safety . . . . . . . . . . . . . . . . . . . . . ii30
7 Safety Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii34
Suggested Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii37
iii
Foreword
There is no reason why anyone ever has to die in a trenching accident. Modern technology has provided
us with a variety of excellent shoring systems and trench shields. The OSHA excavations standard pro-vides
us with a set of clearly written and logical safety rules. Yet every year workers are killed and injured
by cave- ins. Employers and employees can expect N. C. Department of Labor safety inspectors to make the
most detailed, professional inspections possible whenever such accidents occur.
A Guide to the OSHA Excavations Standard examines the standard’s different sections, offering many
illustrations and a safety checklist to help explain how to excavate and work safely in a trench. It also dis-cusses
the new rule concerning excavating and residential construction.
In North Carolina, DOL inspectors enforce the federal Occupational Safety and Health Act through a
state plan approved by the U. S. Department of Labor. The N. C. Department of Labor’s Division of
Occupational Safety and Health offers many educational programs to the public and produces publica-tions,
including this guide, to help inform people about their rights and responsibilities regarding occupa-tional
safety and health.
When looking through this guide, please remember DOL’s mission is greater than just to enforce regu-lations.
An equally important goal is to help people find ways to create safe workplaces. This booklet, like
the other educational materials produced by the N. C. Department of Labor, can help.
Cherie K. Berry
Commissioner of Labor
v
Introduction
The Occupational Safety and Health Administration issued its first Excavation and Trenching
Standard in 1971 to protect workers from excavation hazards. Since then, OSHA has amended the stan-dard
several times to increase worker protection and to reduce the frequency and severity of excavation
accidents and injuries. Despite these efforts, excavation- related accidents resulting in injuries and fatali-ties
continue to occur.
OSHNC has developed this guide in effort to address requirements of the standard, as well as provide
information for equipment operators, workers and all others associated with trenching and excavating to
help recognize hazardous conditions that could result in injury or a fatality. This guide discusses soil com-position
in moderate detail to provide a general overview of the various properties associated with differ-ent
types of soil. A general understanding of the properties of soil is the first step in predicting the behav-ior
of soils in varying condition. Some of the most common types of soil conditions that lead to trench and
excavation failure are also discussed.
Proper trenching operations are necessary to protect the workers from soil collapse. The basic trench-ing
operations that help make a trench safe for workers are described and illustrated. The methods of
shoring installation are also discussed briefly. This document is not intended to be used as a step-by-
step guideline in the excavation process.
OSHA has completely updated the excavations standard, with focus on the existing standard to simpli-fy
many of the existing provisions, add and clarify definitions, eliminate duplicate provisions and ambigu-ous
language, and give employers added flexibility in providing protection for employees. In addition, the
standard provides several new appendices. Appendix A to 1926.652 provides a consistent method of soil
classification. Appendix B to 1926.652 provides sloping and benching requirements. Other appendixes
( appendixes C– F) provide pictorial examples of shoring and shielding devices, timber tables, hydraulic
shoring tables, and selection charts that provide a graphic summary of the requirements contained in the
standard. For more information on the details of proper installation, please refer to the OSHA standard
on excavation ( 29 CFR 1926 Subpart P, which includes 650– 652 and appendixes A– F) and to the
Suggested Readings in this guide.
Scope and Application
OSHA’s revised rule applies to all open excavations made in the earth’s surface, which includes trench-es.
According to the OSHA construction safety and health standards, 1926.650( b), a trench is referred to
as a narrow excavation made below the surface of the ground in which the depth is greater than the
width— the width not exceeding 15 feet ( 4.5 meters). An excavation is any man- made cut, cavity, trench
or depression in the earth’s surface formed by earth removal. This can include excavations for anything,
from cellars to highways. The standard does not apply to house foundation/ basement excavations ( includ-ing
those that become trenches by definition when formwork, foundations or walls are constructed) when
all of the following conditions are present:
• The house foundation/ basement excavation is less than 71/ 2 feet ( 2.5 meters) deep or is benched for at
least 2 feet ( 0.61 meters) horizontally for every 5 feet ( 1.52 meters) or less of vertical height;
• The minimum horizontal width ( excavation face to formwork/ wall) at the bottom of the excavation is
as wide as practicable but not less than 2 feet ( 0.61 meters);
• There is no water, surface tension cracks nor other environmental conditions present that reduce the
stability of the excavation;
• There is no heavy equipment operating in the vicinity that causes vibration to the excavation while
employees are in the excavation;
• All soil, equipment and material surcharge loads are no closer in distance to the top edge of the exca-vation
than the excavation is deep; however, when front end loaders are used to dig the excavations,
the soil surcharge load must be placed as far back from the edge of the excavation as possible, but
never closer than 2 feet ( 0.61 meters).
vii
• Work crews in the excavation are the minimum number needed to perform the work; and
• The work has been planned and is carried out in a manner to minimize the time employees are in
the excavation.
These conditions as applicable to residential construction are restated in another section of this guide,
Residential Contractors and the Excavations Standard.
The standard provides several options for designing trench protection measures. Under certain condi-tions,
the design of the trench protection measures must be prepared by a registered professional engi-neer.
Proper selection and installation of trench protection measures are very important. To comply with the
standard, the employer must have a competent person: “ one who is capable of identifying existing and
predictable hazards in the surroundings or working conditions which are unsanitary, hazardous, or dan-gerous
to employees, and who has authorization to take prompt corrective measures to eliminate them.”
This competent person has critical inspection responsibilities regarding excavations. This person must
inspect every excavation and protective system under his or her care daily, including areas adjacent to
any excavation. Additional inspections must be conducted before starting work and as needed through a
shift, for example, when any hazard- increasing occurrence ( such as a rainstorm) takes place. When the
competent person finds any evidence of a situation that could result in a cave- in, protective system failure
or any other hazardous condition, employees are to be immediately removed from the danger area until
the problem is fixed.
Many companies have established a written policy that outlines specific safe trenching practices in
detail. Such a policy should ensure adequate support for the trench and frequent inspections of the exca-vation
site to detect any change in the soil conditions. When this type of policy is enforced, all employees
understand their responsibilities, which helps to avoid unsafe practices.
This document is not intended to be a guideline for compliance with all pertinent OSHA
regulations but rather an overview of safe practices in trenching operations. Though the
guide is not intended to be inconsistent with OSHA standards, if an area is considered by the
reader to be inconsistent, the OSHA standard should be followed.
viii
1
Common Soil Problems
This guide highlights the requirements in the updated standard for excavation and trenching opera-tions,
provides methods for protecting employees against cave- ins, and describes safe work practices for
employees. A necessary first step in planning the approach to any trenching or other excavation project is
to understand what could go wrong. This understanding can help avoid many of the problems associated
with excavation.
The terms soil and earth are commonly referred to in the excavation process to describe the naturally
occurring materials uncovered on a project. Soil conditions vary from one site to the next. Soil may be
loose or partially cemented, organic or inorganic. However, most soils can be referred to as a mixture or
an accumulation of mineral grains that are not cemented together. An exception is hard rock, which
remains firm after exposure to the elements.
Soil failure is defined as the collapse of part or all of an excavation wall. The most common soil failure
is typically described as an unexpected settlement, or cave- in, of an excavation. Soil sliding is the most
common factor leading to soil failure.
Proper planning and supervision can avoid the unsafe working conditions caused by soil sliding. Unless
such safety precautions have been implemented, sliding soil failure can occur in all types of excavations
( including sloped trenches and excavations with braced trench boxes). See figure 1.
Overview: Soil Mechanics
A number of stresses and deformations can occur in an open cut or trench. For example, increases or
decreases in moisture content can adversely affect the stability of a trench or excavation. The following
diagrams show some of the more frequently identified causes of trench failure.
Tension cracks. Tension cracks usually form at a horizontal distance of one- half to three- quarters times
the depth of the trench, measured from the top of the vertical face of the trench. See figure 2 for addition-al
details.
Figure 1
Sliding Failure
1
Tension Crack
Failed Soil Mass
Figure 2
Tension Crack
Sliding or sluffing may occur as a result of tension cracks, as illustrated in figure 3.
Figure 3
Sliding
Toppling. In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench’s
vertical face shears along the tension crack line and topples into the excavation. See figure 4.
Figure 4
Toppling
2
Tension
Crack
H
.5 to .75 H
Sliding
Toppling
Subsidence and Bulging. An unsupported excavation can create an unbalanced stress in the soil,
which, in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncor-rected,
this condition can cause face failure and entrapment of workers in the trench. See figure 5.
Figure 5
Subsidence and Bulging
Heaving or Squeezing. Bottom heaving or squeezing is caused by the downward pressure created by the
weight of adjoining soil. This pressure causes a bulge in the bottom of the cut, as illustrated in figure 6.
Heaving and squeezing can occur even when shoring or shielding has been properly installed.
Figure 6
Heaving or Squeezing
3
Bulge
Subsidence
Heave Soil
Weight
Boiling is evidenced by an upward water flow into the bottom of the cut. A high water table is one of
the causes of boiling. Boiling produces a “ quick” condition in the bottom of the cut and can occur even
when shoring or trench boxes are used. See figure 7.
Figure 7
Boiling
Unit Weight of Soils refers to the weight of one unit of a particular soil. The weight of soil varies with
type and moisture content. One cubic foot of soil can weigh from 110 pounds to 140 pounds or more, and
one cubic meter ( 35.3 cubic feet) of soil can weigh more than 3,000 pounds.
A safe slope can be defined as the maximum angle of the edge wall or bank of an excavation at which
sliding will not occur. The unique mixtures of the different types of soil ( sand, clay, silt and rock) necessi-tate
different safe slopes from one excavation site to the next.
There are other complicating factors that can result in sliding soil failures. During an excavation, visi-bly
different layers of soil maybe uncovered. Each of those layers may call for different safe slopes. It is
essential to plan your excavation around the most gradual ( rather than steepest) safe slope for all of the
different soil types and layers encountered during the excavation.
Another complicating factor is that soil composition mixtures may vary significantly from one area of
the project to another. During an excavation, as the soil composition changes, the safe slope for trench
wall excavation also changes. Thus, across an excavation site, the slope of the bank may need to be differ-ent
to provide a safe working environment.
Sliding and other modes of failure can also occur in soils that are not densely compacted. For example,
a trench that is made close to a previously dug trench is very unstable. If uncompacted soil is discovered,
the normal safe slope for dense soil will not be enough to prevent sliding. Bracing or further sloping may
be necessary.
If cracks are observed in rocky types of soil, sliding has already occurred. These cracks should signal
that a more gradual slope for excavation is needed because the rocky soil is very susceptible to slides and
other types of failure.
Excavations that have been stable for long periods are also subject to sliding types of failure. After pro-longed
exposure to the elements, the moisture content in the soil may increase. This increase in moisture
may be due to various causes, such as rainfall or a broken water line. The extra soil moisture tends to
speed up sliding soil failures.
Determining the correct safe slope can be quite difficult for certain types of soil. The OSHA standard
has developed a simple method of determining safe excavation bank slopes for different soil types. This
new method will be discussed in more detail in a later section of this document.
4
Water
Table
Boiling
Soil failure can occur for any number of reasons. Factors that increase the chances of soil failure are:
1. excessive vibration
2. surface encumbrances
3. weather conditions
Cave- ins and Protective Support Systems
Excavation workers are exposed to many hazards, but the chief hazard is danger of cave- ins. OSHA
requires that in all excavations employees exposed to potential cave- ins must be protected by sloping or
benching the sides of the excavation, by supporting the sides of the excavation, or by placing a shield
between the side of the excavation and the work area. Designing a protective system can be complex
because of the number of factors involved- soil classification, depth of cut, water content of soil, changes
due to weather and climate, or other operations in the vicinity. The standard, however, provides several
different methods and approaches ( four for sloping and four for shoring, including the use of shields)* for
designing protective systems that can be used to provide the required level of protection against cave- ins.
One method of ensuring the safety and health of workers in an excavation is to slope the sides to an angle
not steeper than one and one- half horizontal to one vertical ( 34 degrees measured from the horizontal).
These slopes must be excavated to form configurations that are in accordance with those for Type C soil
found in appendix B of the standard. A slope of this gradation or less is considered safe for any type of
soil. ( See figure 8).
Figure 8
Type C Soil
Simple Slope Excavation
All simple slope excavations 20 feet ( 6.11 meters) or less in depth must have a maximum allowable
slope of 1.5: 1. A second design method, which can be applied for both sloping and shoring, involves using
tabulated data, such as tables and charts, approved by a registered professional engineer. These data
must be in writing and must include sufficient explanatory information to enable the user to make a
selection, including the criteria for determining the selection and the limits on the use of the data. At
least one copy of the information, including the identity of the registered professional engineer who
approved the data, must be kept at the worksite during construction of the protective system. Upon com-pletion
of the system, the data may be stored away from the jobsite, but a copy must be made available
upon request to OSHNC. Contractors may also use a trench box or shield that is either designed or
approved by a registered professional engineer or is based on tabulated data prepared or approved by a
registered professional engineer. Timber, aluminum or other suitable materials may also be used. OSHA
standards permit the use of a trench shield ( also known as a welder’s hut) as long as the protection it pro-vides
is equal to or greater than the protection that would be provided by the appropriate shoring system.
( See figure 9.)
5
* See appendix F to the standard for a complete overview of all options.
20' Maximum
11/ 2
1
Figure 9
Trench Shield
The standard does not require the installation and use of a protective system when an excavation is
made entirely in stable rock or is less than 5 feet deep ( 1.52 meters) and a competent person has exam-ined
the ground and found no indication of a potential cave- in.
Vibrations
Any large, heavy movement near an excavation results in vibration of the surrounding soils. This
movement can result in soil failure. Moving machinery, nearby traffic, pile driving and blasting all cause
vibration in surrounding soils.
Vibration- related soil failures can occur in all types of soil. However, certain types of soils are more sus-ceptible
to vibration failures than others. For example, sandy soils tolerate less vibration than clay soils.
Since actual soil conditions may be a mixture of more than one soil type, it is better to play it safe when
planning the slope of an excavation. Figure 10 shows typical situations where vibrations can result in soil
failure.
Figure 10
Two Examples of Vibration Failures
6
Strut
Knife Edge
Sidewall
Soil affected by the movement
of the crane and susceptible to
sliding
Soil affected by the movement
of the truck and susceptible to
sliding
Surface Encumbrances
Heavy loads such as large equipment, heavy materials or large spoil piles can be too heavy for the soil
to support, resulting in a cave- in. These loads are referred to as surface encumbrances. They pose differ-ent
types of dangers ( see figure 3). For example, large spoil piles may hide tension cracks that would oth-erwise
signal that a sliding soil failure may occur.
Figure 11
Surface Encumbrances
Existing site features such as buildings, curbs, trees, utility poles and other structures adjoining the
excavation area may be considered as types of surface encumbrances. These extra loads can place more
stress on the sides of an excavation than the walls can safely carry. Shoring, bracing, underpinning or
some combination of safety measures should be provided, as necessary, to protect workers and to prevent
movement of the soil beneath the adjacent load.
In cases where the space is limited and heavy loads must be located near an excavation, the trench
walls must be braced or shored as needed to safely support this extra weight.
Weather Conditions
Weather is an important factor in determining soil conditions. More importantly, changing weather
conditions may signal a change in the pressures exerted by the soil on the side walls of a trench.
Excess water from rain or melting snow interacts with the soil, increasing the pressure on the excava-tion
and shoring system. For instance, a rainstorm can turn a stable trench wall that required only light
bracing into a mass of loose soil that requires heavy bracing.
Freezing usually indicates a rather stable ground condition, unless the frost line is exceeded during exca-vation.
The frost line phenomenon is depicted in figure 12. If you excavate or shore frozen ground, be aware
that another potential problem exists— thawing. A sudden thaw can be as dangerous as a rainstorm.
Figure 12
Region of Soil Freezing
Excessively dry conditions can also be dangerous. As moisture content decreases, some dry soils lose
their ability to stick together. This lack of cohesion may result in a sliding type of soil failure. In many of
the situations described above, dewatering or extra shoring may be required as necessary to ensure the
safety of your workers. See figures 16 and 17 in part 2 for more information about dewatering.
7
Tension Crack
These loads not only create
forces too heavy for the soil
to carry, but also cover up
and signs of failure.
=
Ground level
Water level
Frost line
The water above the frost line can freeze
The water below the frost line does not freeze
2
Soil Types and Pressures
Soil Characteristics
The OSHA excavations standard recognizes and allows a variety of soil classification systems under
certain conditions. A special simple soil classification system used by OSHA for excavation planning and
protection is included in the standard. If that classification system is strictly followed, trench protection
systems can be designed for many situations without the approval of a registered professional engineer.
In the soil classification system used by OSHA, the terms used to identify soil types are drawn largely
from another system, commonly used for construction in North Carolina, called the United Soil
Classification System. Both systems are based upon the engineering properties of soils and are concise
and easily associated with actual soil behavior. The OSHA system can be applied in the laboratory or the
field. The terms used for classification are based upon the soil particles, including the quantity of the var-ious
sizes of soil particles and the qualities or characteristics of the very fine grains. The principal types
of soil may be divided into two general classes according to grain size. Coarse- grained soils are gravel and
sand. Fine- grained soils are silt and clay.
The composition or texture of a soil is a critical factor in its stability. The more cohesive the soil parti-cles,
the more the entire soil mass tends to stick together rather than crumble. However, it is important
to remember the time element involved in cuts. If an excavated cut is to be left open for long periods of
time, cohesive forces may not withstand exposure to weather conditions. When fresh fill dirt is not prop-erly
compacted, subsequent excavations in the same area result in almost no cohesion properties; thus, a
greater width may be required to maintain a stable slope.
The soil found at a site is usually a mixture of one or more of the basic types listed below. From the
amounts of each soil type blended together to form the actual soil conditions, descriptive soil terms are
combined in the order of lowest content to highest content. For example, soil classified as “ silty clay” is a
mixture of mostly clay with noticeable but lesser amounts of silt. The single term “ loam” is used to
describe a mixture of clay, sand and silt.
The types of soil found most often include:
Soil Type Characteristics of Soil Type
Clay Clay is a very, very fine- grained soil. In general, water moves very slowly through clay.
Large amounts of rainfall may pond on the surface and evaporate before being absorbed.
Sand Sand is a granular soil. The shape of individual grains may be round or angular. Sandy
soils tend to have large pores, allowing faster groundwater absorption. In most situations,
sandy soil is the easiest to compact with vibration.
Silt Silt has properties intermediate between fine sand and clay. Silt is the most sensitive to
changes in soil moisture content. Silt tends to crumble with drying.
Determination of Soil Type
By grouping different types of soils ( described above) according to requirements for safe excavation, the
excavation standard has defined four soil classifications ( provided below). For a detailed explanation of
OSHA classification system, please see appendix A of the excavation standard. OSHA categorizes soil and
rock deposits into four types, A through D, as follows:
Stable Rock is natural solid mineral matter that can be excavated with vertical sides and remain
intact while exposed. It is usually identified by a rock name such as granite or sandstone. Determining
whether a deposit is of this type may be difficult unless it is known whether cracks exist and whether or
not the cracks run into or away from the excavation.
8
Type A soils are cohesive soils with an unconfined compressive strength of 1.5 tons per square foot
( 144 kPa) or greater. Examples of Type A cohesive soils are clay, silty clay, sandy clay, clay loam and, in
some cases, silty clay loam and sandy clay loam. ( No soil is Type A if it is fissured, is subject to vibration
of any type, has previously been disturbed, is part of a sloped, layered system where the layers dip into
the excavation on a slope of four horizontal to one vertical or greater, or has seeping water.)
Type B soils are cohesive soils with an unconfined compressive strength greater than 0.5 tons per
square foot ( 48 kPa) but less than 1.5 ( 144 kPa). Examples of Type B soils are angular gravel; silt; silt
loam; previously disturbed soils unless otherwise classified as Type C; soils that meet the unconfined
compressive strength or cementation requirements of Type A soils but are fissured or subject to vibration;
dry unstable rock; and layered systems sloping into the trench at a slope less than four horizontal to one
vertical ( only if the material would be classified as a Type B soil).
Type C soils are cohesive soils with an unconfined compressive strength of 0.5 tons per square foot ( 48
kPa) or less. Type C soils include granular soils such as gravel, sand and loamy sand, submerged soil, soil
from which water is freely seeping, and submerged rock that is not stable. Also included in this classifica-tion
is material in a sloped, layered system where the layers dip into the excavation or have a slope of
four horizontal to one vertical or greater.
Layered geological strata. Where soils are configured in layers, i. e., where a layered geologic struc-ture
exists, the soil must be classified on the basis of the soil classification of the weakest soil layer. Each
layer may be classified individually if a more stable layer lies below a less stable layer, i. e., where a Type
C soil rests on top of stable rock.
The standard also contains other important criteria that must be examined to classify soils properly.
Important: 1. The laboratory testing process and compressive strength calculations must be conducted
under the direction of a registered professional engineer. The OSHA standard requires that the excava-tion
protection system be designed by a registered professional engineer when the depth of the excavation
exceeds 20 feet or where unusual site conditions exist. 2. The manual field testing alternative permitted
under the standard does not require the approval of a registered professional engineer under certain spe-cific
conditions. However, at least one visual test and one manual test are required to classify soil accord-ing
to the OSHA method. The specific manual and visual field tests are listed and described in the stan-dard.
Test Equipment and Methods for Evaluating Soil Type
Many kinds of equipment and methods are used to determine the type of soil prevailing in an area, as
described below.
Pocket Penetrometer. Penetrometers are direct- reading, spring- operated instruments used to deter-mine
the unconfined compressive strength of saturated cohesive soils. Once pushed into the soil, an indi-cator
sleeve displays the reading. The instrument is calibrated in either tons per square foot or kilograms
per square centimeter. However, penetrometers have error rates in the range of ± 20– 40 percent.
1. Shearvane ( Torvane). To determine the unconfined compressive strength of the soil with a shear-vane,
the blades of the vane are pressed into a level section of undisturbed soil, and the torsional
knob is slowly turned until soil failure occurs. The direct instrument reading must be multiplied by
2 to provide results in tons per square foot or kilograms per square centimeter.
2. Thumb Penetration Test. The thumb penetration procedure involves an attempt to press the thumb
firmly into the soil in question. If the thumb makes an indentation in the soil only with great diffi-culty,
the soil is probably Type A. If the thumb penetrates no further than the length of the thumb
nail, it is probably Type B soil, and if the thumb penetrates the full length of the thumb, it is Type C
soil. The thumb test is subjective and is therefore the least accurate of the three methods.
3. Dry Strength Test. Dry soil that crumbles freely or with moderate pressure into individual grains is
granular. Dry soil that falls into clumps that subsequently break into smaller clumps ( and the small-er
clumps can be broken only with difficulty) is probably clay in combination with gravel, sand or silt.
9
If the soil breaks into clumps that do not break into smaller clumps ( and the soil can be broken only
with difficulty), the soil is considered unfissured unless there is visual indication of fissuring.
Plasticity or Wet Thread Test. This test is conducted by molding a moist sample of the soil into a ball
and attempting to roll it into a thin thread approximately 1/ 8 inch ( 3 millimeters) in diameter ( thick) by 2
inches ( 50 millimeters) in length. The soil sample is held by one end. If the sample does not break or tear,
the soil is considered cohesive.
Visual Test. A visual test is a qualitative evaluation of conditions around the site. In a visual test, the
entire excavation site is observed, including the soil adjacent to the site and the soil being excavated. If
the soil remains in clumps, it is cohesive; if it appears to be coarse- grained sand or gravel, it is considered
granular. The evaluator also checks for any signs of vibration.
During a visual test, the evaluator should check for crack- line openings along the failure zone that
would indicate tension cracks, look for existing utilities that indicate that the soil has previously been
disturbed, and observe the open side of the excavation for indications of layered geologic structuring.
The evaluator should also look for signs of bulging, boiling or sluffing, as well as for signs of surface
water seeping from the sides of the excavation or from the water table. If there is standing water in the
cut, the evaluator should check for “ quick” conditions. In addition, the area adjacent to the excavation
should be checked for signs of foundations or other intrusions into the failure zone, and the evaluator
should check for surcharging and the spoil distance from the edge of the excavation.
Earth Pressures
An appreciation of the need for shoring can be based on an understanding of earth pressures. The
amount of earth pressure exerted upon the side wall of any excavation depends upon the weight and
depth of the soil that it supports.
Earth pressure distributions vary with the type of soil, depth of excavation and moisture conditions.
Example distributions are shown in figure 13.
The center of the earth pressures is normally found between one- half and two- thirds of the depth of a
simple excavation. However, additional earth pressures result from surface encumbrances and differ-ences
in soil layer cohesiveness. All of these factors influence the actual center of pressure at an excava-tion
site.
Figure 13
Pressure Distributions on the Side Walls of an Excavation
10
Sand and Granular
Soils
Soft Clay
Trench Wall
Pressure
Stiff Clay
As a general rule of thumb, the center of pressure will be at a lower depth when cohesion is poor ( as in
fresh fill dirt, water- bearing sand or loose ground). Where cohesion is high, the center of pressure will be
higher ( as in good compact soil). The location of the center of pressure can change after a cut is made
unless support is provided to prevent earth movement.
Regardless of the soil type, as the depth of the trench increases, the magnitude of pressures on the full
height of the excavation also increases. The presence of ground water adds hydrostatic pressure against
the walls of the trench as shown in figure 14.
Figure 14
Water Pressure
Effects of Water and Remedies
The natural water table can cause many types of problems. For example, trenches excavated below the
natural water table in sandy soils and soft clay are highly susceptible to heaving, as illustrated in figure
15. Heaving is the seepage of water at the bottom of the trench causing the soil to be pushed upward.
This heaving is a signal that a failure may occur.
Figure 15
Heaving
11
Ground water
table
Water
Pressure
Added effect of water pressure
Water Table
Dry Soil
Granular Saturated Soil
Heave
Granular
Soil
Water Movement
Wet conditions at the bottom of a trench may present another problem. If the bottom of the trench
begins to puff and bubble and the earth rises, a quicksand condition is occurring. This is also a signal
that a failure may occur.
If heaving or quicksand conditions are expected, dewatering should be considered prior to beginning an
excavation. Dewatering drastically reduces the presence of water and the additional pressure it causes.
Without dewatering, heavier timbers would be needed to support the extra pressures caused by the
water. The two most frequently used dewatering systems are well- points and sump pumps.
The well- point system ( illustrated in figure 16) is a very popular method of dewatering. Located on a
line 2 or 3 feet, or further, behind the sheeting, well- points are inserted to the depth of the excavation
with spacing between the well- points varying from 3 to 8 feet.
Figure 16
Well- Point
Well- points are pipes with a point at the lower end and a screen or filter over perforations along 3 or 4
feet of the lower ends of the pipes. There are two types of well- points:
those driven with a maul
those that are jetted in
The selection of the size of the well- points and the required spacing are based upon site conditions and
the type of excavation to be accomplished.
Above the ground, well- point pipes are connected by piping to a high- capacity pump. Pumping keeps
the water level below the bottom of the excavation so that only a moist soil condition will be encountered
within the excavation.
The well- point system should have a capacity sufficient to remove any inflow of water as quickly as it
occurs. The depth limit of this method’s practical effectiveness is approximately 15– 20 feet, although the
theoretical limit is just under 34 feet since the method depends upon pumping suction. Greater depths
can be achieved by arranging well- points into two or more vertical stages, or by deep- well pumping, that
is, locating the pump at a lower elevation.
Dewatering does not permit any substantial excavation without providing ground support. Although
the dewatered soil will usually be firmer than it was before dewatering, working conditions may still be
unsafe. Shoring, or bank walls at a safe slope, should be used in dewatered ground in the same manner
as in any other excavation.
12
Header
Sand and
gravel filter
Inflow pipe
covered with
filtering screen
Head
Jetting Holes
Lowered water table
Continuous
Sheeting
Ground
Trench
braces
Riser
The second common type of dewatering system is the sump pump as depicted in figure 17.
Figure 17
Sump Pump
Sump pumping, as contrasted with well- pointing, has several advantages. Sump pumps:
can be installed quickly by inexperienced labor
require less space and cause less interference on the site
can be added or removed easily to meet required pumping capacity
can be started by simply switching on the power supply because no balancing or turning is required
do not freeze in cold weather because of the fast, high volume flow of water
may be removed from one sump and used elsewhere if needed
usually cost less than well- points
13
Discharge pipe to a distant point
Pump
Suction pipe
Wire mesh
Coarse sand and
gravel filter
Bracing
Lowered water table
Sides supported by
sheeting until filter and
bracing are installed
3
Methods of Protection
Several methods of protecting workers in trench excavations are available. Some methods are pre-ferred
over others depending on the site specifics and circumstances. The various protection methods
available include:
1. sloping and benching
2. shoring ( spaced sheeting, close sheeting)
3. trench shield
4. other occasionally used systems
Each of these techniques is introduced briefly below and will be discussed in more detail in the pages
that follow.
Materials and Equipment
The employer is responsible for the safe condition of materials and equipment used for protective sys-tems.
Defective and damaged materials and equipment can result in the failure of a protective system
and cause excavation hazards. To avoid possible failure of a protective system, the employer must ensure
that:
1. materials and equipment are free from damage or defects,
2. manufactured materials and equipment are used and maintained in a manner consistent with the
recommendations of the manufacturer and in a way that will prevent employee exposure to hazards,
and
3. while in operation, damaged materials and equipment are examined by a competent person to deter-mine
if they are suitable for continued use. If materials and equipment are not safe for use, they
must be removed from service. These materials cannot be returned to service without the evaluation
and approval of a registered professional engineer.
Sloping and Benching
One method of ensuring the safety and health of workers in a trench or excavation is to slope the sides
of the trench in accordance with OSHA standards. Figure 18 illustrates sloping and benching alterna-tives
that are permitted for certain soil types. The safe slope for the banks of an excavation varies with
different soil types, as discussed earlier, and must be determined on each individual project.
Figure 18
Sloping and Benching
14
20' Maximum
4' Maximum
1
1
Single Bench
This example is allowed only in cohesive Type B soil
Spaced Sheeting
Another popular method of protection is called spaced sheeting. It is also referred to as spot shoring.
This method, shown in figure 19, involves placing spaced timber shores, bracing, trench jacks, piles or
other materials in a manner strong enough to resist the pressures surrounding the excavation. Sheeting
consists of vertical planks used around the boundary of the proposed excavation. Horizontal braces
extend between the vertical planks to support the sheeting. The horizontal trench braces may be wooden
or telescoping metal pipes. The metal braces are typically used when the width of the trench exceeds 5
feet. It is important to remember that all materials selected for use must be in good condition.
Figure 19
Spaced Sheeting
Close Sheeting
This method involves the placement of continuous solid sheeting along the entire length of a trench
excavation. An example is shown in figure 20. The same types of materials used in spaced sheeting can be
used in close sheeting. Timber sheeting or steel sheet piles may be selected for use depending on the cir-cumstances.
As a general rule of thumb, steel sheeting becomes more cost effective when the depth of a
planned excavation exceeds 15 feet. Each of the major components of this system has been labeled in fig-ure
20. Cleats may also be used to fasten the struts to the wales and prevent slipping or falling out.
15
Sheeting
Wales with jack screws
Figure 20
Close Sheeting
Trench Shield
Contractors also may use a trench shield, a prefabricated movable structure often composed of steel
plates welded to a heavy steel frame ( see figure 21). Some trench shields are composed of aluminum or
fiberglass. OSHA standards permit the use of a trench shield as long as the protection it provides is equal
to or greater than the protection that would be provided by the appropriate shoring system. Employees
must know to work only within the protection of the shield. Also, if a slide starts, workers must know
that they should not run out of the shield into the path of the slide.
16
Jack screw Continuous sheet piling
Wale
Metal brace or strut
Figure 21
Trench Shield
Other Systems
Some other systems that are used occasionally include:
• Freezing the moist or saturated soil by the circulation of low- temperature brine through piping dri-ven
into the soil
• Injection of chemical or other grouting into the soil to solidify and fill cracks and space surrounding
the individual soil particles to solidify the soil mass
17
4
Installation and Removal of Protective Systems
To ensure the safety of the workers and the integrity of the job, it is essential to install the various
types of trench protection properly. The standard requires the following procedures for the protection of
employees when installing support systems:
• securely connect members of support systems,
• safely install support systems,
• never overload members of support systems, and
• install other structural members to carry loads imposed on the support system when temporary
removal of individual members is necessary.
In addition, the standard permits excavation of 2 feet ( 0.61 meters) or less below the bottom of the
members of a support or shield system of a trench if:
1. the system is designed to resist the forces calculated for the full depth of the trench, and
2. there are no indications, while the trench is open, of a possible cave- in below the bottom of the sup-port
system.
Also, the installation of support systems must be closely coordinated with the excavation of trenches.
As soon as work is completed, the excavation should be back- filled as the protective system is dismantled.
After the excavation has been cleared, workers should slowly remove the protective system from the bot-tom
up, taking care to release members slowly.
Sloping and Benching
As mentioned earlier, one method of trench protection can be accomplished by sloping the sides of the
trench to the safe angle specified by OSHA excavation standards. The trench is sloped on both sides.
The safe angle to slope the sides of an excavation varies with different kinds of soil and must be deter-mined
on each individual project. When an excavation has a high water table condition, silty material or
loose boulders, or when it is being dug in areas where erosion, deep frost or sliding is apparent, the safe
angle is more gradual ( that is, flatter) ( see figure 22).
Figure 22
Sloping
Problems Associated With Sloping and Benching
1. The spoil accumulated from digging a trench must be placed above and away from the side walls of
the excavation. Otherwise, the weight of the spoil might create an unsafe condition. OSHA requires
that the spoil be kept 2 feet or more from the edge of the excavation or prevented from falling or
rolling into the excavation by the use of retaining devices. This procedure usually requires a wide
storage area.
18
Determine safe slope
from standard
Too steep
2 ft. minimum,
greater distance
is preferred
Spoil
2. Wide excavation areas can expose footings or cause damage to the walls of an adjacent structure
and thereby pose additional hazards to employees.
3. Wide excavation areas can expose or place utilities ( such as electric power, telephone, water, gas,
storm drain or sewer lines) above the angle for a safe slope, causing the unsupported collapse and
failure of the utility line.
4. Wide areas of excavation require the use of large equipment. There may also be hazards in the
movement of the equipment across a larger excavation. For example, excavation under or adjacent
to electric power lines creates a serious hazard to workers and the public.
5. To prevent the collapse of an unsupported bench in an excavation 8 feet or less in depth, the allow-able
height of a bench at the base of an excavation must be 31/ 2 feet or less. The collapse of one bench
can cause a lower bench to fail in a situation where multiple benches have been excavated. For Type
A soil, for example, the standard requires that multiple benches have an overall slope ( from the top
of one side of an excavation to the bottom) of 3/ 4 horizontal to 1 vertical ( see figure 23).
Figure 23
Unsupported Vertically Sided Lower Portion— Maximum Eight Feet in Depth
The contractor should first make a determination of the soil types at the excavation site using the soil
classification system used by OSHA or one of the other acceptable methods described in the standard.
Next, the contractor should consider potential sloping and benching problems, such as those described
above. Finally, after considering all other protection that may be necessary to ensure safe working condi-tions,
the contractor can determine if sloping ( and possibly benching) is the best method to use at that
site. Figure 24 defines the maximum allowable slopes for excavations less than 20 feet deep. This figure is
shown as table B- 1 in the standard.
Figure 24
Maximum Allowable Slopes
19
8' Max
3/ 4
1
31 / 2 ' Max
All excavations more than 8 feet but not more than 12 feet in depth with unsupported
vertically sided lower portions must have a maximum allowable slope of 1: 1 and a
maximum vertical side of 31/ 2 feet.
Soil or rock type Maximum allowable slopes ( H: V) 1
for excavations less than 20 feet deep3
Stable rock Vertical ( 90°)
Type A2 3⁄ 4: 1 ( 53°)
Type B 1: 1 ( 45°)
Type C 11⁄ 2: 1 ( 34°)
Notes:
1 Numbers shown in parentheses next to maximum allowable slopes are angles
expressed in degrees from the horizontal. Angles have been rounded off.
2 A short- term maximum allowable slope of 1⁄ 2H: 1V ( 63°) is allowed in excava-tions
in Type A soil that are 12 feet ( 3.67 m) or less in depth. Short- term maxi-mum
allowable slopes for excavations greater than 12 feet ( 3.67 m) in depth
shall be 3⁄ 4H: 1V ( 53°).
3 Sloping or benching for excavations greater than 20 feet deep shall be
designed by a registered professional engineer.
Benching. There are two basic types of benching: simple and multiple. The type of soil determines the
horizontal to vertical ratio of the benched side. As a general rule, the bottom vertical height of the trench
must not exceed 4 feet ( 1.2 meters) for the first bench. Subsequent benches may be up to a maximum of 5
feet ( 1.5 meters) vertical in Type A soil and 4 feet ( 1.2 meters) in Type B soil to a total trench depth of 20
feet ( 6.0 meters). All subsequent benches must be below the maximum allowable slope for that soil type.
For Type B soil the trench excavation is permitted in cohesive soil only.
Figure 25
Excavation Made in Type B Soil
The OSHA excavations standard provides four options for design and installation of a sloped excava-tion.
See figure 26.
Option 1. Excavations must be sloped at an angle not steeper than 11⁄ 2 horizontal to 1 vertical ( 34
degrees). The exact configuration of the sides must also conform to other requirements ( described in
appendix B of the standard) for sloping of Type C soil.
Option 2. The design and installation may be performed in accordance with the soil classification sys-tem
used by OSHA ( appendix A of the standard), followed by the appropriate sloping requirements listed
in appendix B of the standard.
Option 3. The excavation design and installation of the sloping approach can be performed in accor-dance
with tabulated data ( such as tables and charts) approved for use by a registered professional engi-neer.
Option 4. Sloping and benching systems not utilizing options 1, 2 or 3 ( above) must be approved by a
registered professional engineer.
Additional considerations and other important requirements for proper selection of a sloping system
are contained in the standard.
20
20' Maximum
4' Maximum
1
1
Type B Soil
Single Bench Excavation
( Permitted in cohesive soil only)
Type B Soil
Multiple Bench Excavation
( Permitted in cohesive soil only)
20' Maximum
4' Maximum
4' Maximum
1
1
Figure 26
Slope Configurations: Excavations in Layered Soils
21
20' Maximum
3/ 4
1
Type A Soil
Simple Slope Excavation
20' Maximum
1
1
Type B Soil
Simple Slope Excavation
20' Maximum
11/ 2
1
Type C Soil
Simple Slope Excavation
Type A Soil
over
Type A Type C Soil
Type C
1
1
11/ 2
11/ 2
Type B Soil
over
Type B Type C Soil
Type C
1
1
11/ 2
11/ 2
Type A Soil
over
Type B Soil
Type B
Type A
1
1
1
1
Type C Soil
over
Type C Type B Soil
Type B
1
1
1
11/ 2
Type B Soil
over
Type A Soil
Type B
Type A
1
1
1
3/ 4
Type C Soil
over
Type C Type A Soil
Type A
1
1
11/ 2
3/ 4
In case of an emergency, workers must be able to leave the trench quickly. According to OSHA regula-tions,
when employees are required to be in trenches 4 feet deep or more, adequate means of exit, such as
a ladder or steps, must be provided and located so as to require no more than 25 feet of lateral travel.
Support Systems, Shield Systems and Other Protective Systems
When a trench is excavated, employees who are to work in the excavation must be protected from cave-ins.
In addition to the bracing described earlier, the contractor should consider excavating a wider area
than the minimum necessary. Such additional excavation provides a more comfortable working environ-ment
in the trench. In addition, this extra working area may provide a means for workers to escape unex-pected
crises, such as falling objects or debris. Another common sense strategy for safety in trenches
requires managers to reduce risk by limiting the number of workers in the trench at all times. The only
workers allowed in the trench should be those who are absolutely needed to perform the task at hand.
As the trench is backfilled, the braces and planks can be removed for reuse. If installed and removed prop-erly,
vertical planks and trench braces may be used several times. Spaced sheeting is shown in figure 27.
Figure 27
Spaced Sheeting
Close Sheeting
When the soil is unstable, the excavation should be supported along the entire area of the exposed
trench walls. This can be done by installing continuous sheeting that extends the full depth of the trench,
as depicted in figure 28. Tables have been provided in the standard for the selection of timber sizes for
various excavation depths and widths. The timber sizes listed in the tables are for general reference only
and are not adequate for all soil conditions. Be sure to read the information that explains how to refer-ence
the tables properly.
22
Trench jack
Uprights
Very unstable soil conditions will require the sheeting to be driven prior to digging, with the bracing
installed as excavation proceeds. If steel sheeting is used, a pile driver will be necessary for installation.
For wooden sheeting, a pneumatic hammer is often used. The sheeting should extend one foot above the
surrounding ground to help prevent pipe, tools or equipment from falling into the trench.
Figure 28
Close Sheeting
Trench Shields
OSHA regulations allow the use of portable trench boxes or shields in lieu of fixed shoring systems as
long as an equivalent or greater level of employee protection is provided. In deeper trenches, the trench
shield approach is often the safest. The shields may be made with top and bottom sections for adaptabili-ty
to deep and shallow excavations.
Trench boxes or shields:
may be production type or custom made of steel, aluminum or other equivalent material
must be regularly inspected and properly maintained
must be properly used under the direction of a competent person
Trench boxes are different from shoring because, instead of shoring up or otherwise supporting the
trench face, they are intended primarily to protect workers from cave- ins and similar incidents. The exca-vated
area between the outside of the trench box and the face of the trench should be as small as possible.
The space between the trench boxes and the excavation side are backfilled to prevent lateral movement of
the box. Shields may not be subjected to loads exceeding those which the system was designed to with-stand.
23
Braces
Sheet piles
Cleats
Stringers
Combined use. Trench boxes are generally used in open areas, but they also may be used in combina-tion
with sloping and benching. The box should extend at least 18 inches ( 0.45 meters) above the sur-rounding
area if there is sloping toward excavation. This can be accomplished by providing a benched
area adjacent to the box.
Earth excavation to a depth of 2 feet ( 0.61 meters) below the shield is permitted, but only if the shield
is designed to resist the forces calculated for the full depth of the trench and there are no indications
while the trench is open of possible loss of soil from behind or below the bottom of the support system.
Conditions of this type require observation on the effects of bulging, heaving and boiling as well as sur-charging,
vibration, adjacent structures, etc., on excavating below the bottom of a shield. Careful visual
inspection of the conditions mentioned above is the primary and most prudent approach to hazard identi-fication
and control.
Figure 29
Slope and Shield Configurations
Examples of Other Systems
Many shoring systems are introduced to the marketplace each year. The structural supporting mem-bers
are typically made of wood or metal. For most of these systems, the horizontal trench jacks or braces
are activated using air or hydraulic pumps. These types of systems are illustrated in figure 30.
24
Type A Soil
Supported or shielded
Vertically sided lower portion
Support or shield
system
20' Maximum
1
18" Minimum
3/ 4
Type B Soil
Supported or shielded
Vertically sided lower portion
Support or shield
system
20' Maximum
1
18" Minimum
1
Type C Soil
Supported or shielded
Vertically sided lower portion
Support or shield
system
20' Maximum 1
18" Minimum
11/ 2
Figure 30
Air Shoring or Hydraulic Shoring
Shoring Types
Shoring is the provision of a support system for trench faces used to prevent movement of soil, under-ground
utilities, roadways and foundations. Shoring or shielding is used when the location or depth of the
cut makes sloping back to the maximum allowable slope impractical. Shoring systems consist of posts,
wales, struts and sheeting. There are two basic types of shoring, timber and aluminum hydraulic. See fig-ures
31 and 32.
Hydraulic shoring. The trend today is toward the use of hydraulic shoring, a prefabricated strut
and/ or wale system manufactured of aluminum or steel. Hydraulic shoring provides a critical safety
advantage over timber shoring because workers do not have to enter the trench to install or remove
hydraulic shoring. Other advantages of most hydraulic systems are that they:
• Are light enough to be installed by one worker;
• Are gauge- regulated to ensure even distribution of pressure along the trench line;
• Can have their trench faces “ preloaded” to use the soil’s natural cohesion to prevent movement; and
�� Can be adapted easily to various trench depths and widths.
All shoring should be installed from the top down and removed from the bottom up. Hydraulic shoring
should be checked at least once per shift for leaking hoses and/ or cylinders, broken connections, cracked
nipples, bent bases, and any other damaged or defective parts.
25
18" Maximum
Vertical
spacing
4' Maximum
2' Maximum
Vertical rail
Hydraulic cylinder
Aluminum hydraulic shoring
Examples of trench jacks
used in pneumatic ( air)
and hydraulic shoring
Figure 31
Shoring Variations: Typical Aluminum Hydraulic Shoring Installations
Pneumatic shoring works in a manner similar to hydraulic shoring. The primary difference is that
pneumatic shoring uses air pressure in place of hydraulic pressure. A disadvantage to the use of pneu-matic
shoring is that an air compressor must be on site. Air shoring involves using compressed air
instead of hydraulic fluid to expand the trench jacks into position. Using the air type of system, pins are
put in place to lock the jacks when a desired level of stability has been achieved. For the removal of this
trenching system, air is again injected into the jacks to extend them, allowing the pin to be removed.
These types of jacks are popular since they are cleaner than hydraulic jacks and there is no danger from
the leakage of fluids or other lubrication.
1. Screw jacks. Screw jack systems differ from hydraulic and pneumatic systems in that the struts of a
screw jack system must be adjusted manually. This creates a hazard because the worker is required
to be in the trench in order to adjust the strut. In addition, uniform “ preloading” cannot be achieved
with screw jacks, and their weight creates handling difficulties.
2. Single- cylinder hydraulic shores. Shores of this type are generally used in a water system, as an
assist to timber shoring systems, and in shallow trenches where face stability is required.
26
Vertical Aluminum Hydraulic Shoring
( Spot Bracing)
Vertical Aluminum Hydraulic Shoring
( With Plywood)
Vertical Aluminum Hydraulic Shoring
( Stacked)
Aluminum Hydraulic Shoring Waler System
( Typical)
Vertical
Roll
Vertical
Roll
Vertical
Roll
Plywood
Hydraulic
Cylinder
Hydraulic
Cylinder
Hydraulic
Cylinder
Hydraulic
Cylinder
Upright
Sheeting
Wale
3. Underpinning. This process involves stabilizing adjacent structures, foundations and other intru-sions
that may have an impact on the excavation. As the term indicates, underpinning is a proce-dure
in which the foundation is physically reinforced. Underpinning should be conducted only under
the direction of and with the approval of a registered professional engineer.
Figure 32
Shoring Variations
The excavations standard provides four options for the design of support systems, shield systems and
other protective systems. A summary of the options follows.
Option 1. The soil conditions encountered at the site must first be classified using the soil classification
system used by OSHA ( appendix A of the standard). Based on the soil classification and other project con-ditions,
the contractor may select a timber shoring system using the information contained in appendix C
or an aluminum hydraulic shoring system using the information contained in appendix D. The informa-tion
contained in appendix D should only be used when the manufacturer’s data for an hydraulic system
are not available. The information in appendix D includes tables that detail the maximum vertical and
horizontal spacings that may be used with various aluminum member sizes and various hydraulic cylin-der
sizes.
Option 2. The manufacturer’s tabulated data provided with commercially available support systems,
shield systems or other protective systems may be used if jobsite conditions and methods of use are in
strict accordance with the design intent of the system.
Option 3. Other tabulated data, such as tables and charts, prepared by a registered professional engi-neer
for use under the conditions at the site may be used for the design of support systems, shield sys-tems
and other protective systems.
Option 4. Support systems, shield systems and other protective systems not using options 1, 2 or 3
( above) must be approved by a registered professional engineer.
There is other important information in the standard that must be reviewed to execute one of these
design options correctly.
Other Hazards
Falls and Equipment
In addition to cave- in hazards and secondary hazards related to cave- ins, there are other hazards from
which workers must be protected during excavation- related work. These hazards include exposure to
falls, falling loads and mobile equipment. To protect employees from these hazards, OSHA requires the
employer to take the following precautions:
27
Upright Sheeting
Screw
jack
Wale Pneumatic/ hydraulic jacks
Screw jack
• Keep materials or equipment that might fall or roll into an excavation at least 2 feet ( 0.61 meters)
from the edge of excavations, or have retaining devices, or both.
• Provide warning systems such as mobile equipment, barricades, hand or mechanical signals, or stop
logs to alert operators of the edge of an excavation. If possible, keep the grade away from the excava-tion.
• Provide scaling to remove loose rock or soil or install protective barricades and other equivalent pro-tection
to protect employees against falling rock, soil or materials.
• Prohibit employees from working on faces of sloped or benched excavations at levels above other
employees unless employees at lower levels are adequately protected from the hazard of falling,
rolling or sliding material or equipment.
• Prohibit employees under loads that are handled by lifting or digging equipment. To avoid being
struck by any spillage or falling materials, require employees to stand away from vehicles being
loaded or unloaded. If cabs of vehicles provide adequate protection from falling loads during loading
and unloading operations, the operators may remain in them.
Water Accumulation
The standard prohibits employees from working in excavations where water has accumulated or is
accumulating unless adequate protection has been taken. If water removal equipment is used to control
or prevent water from accumulating, the equipment and operations of the equipment must be monitored
by a competent person to ensure proper use.
OSHA standards also require that diversion ditches, dikes or other suitable means be used to prevent
surface water from entering an excavation and to provide adequate drainage of the area adjacent to the
excavation. Also, a competent person must inspect excavations subject to runoffs from heavy rains.
Hazardous Atmospheres
Under this provision, a competent person must test excavations greater than 4 feet ( 1.22 meters) in
depth as well as ones where oxygen deficiency or a hazardous atmosphere exists or could reasonably be
expected to exist, before an employee enters the excavation. If hazardous conditions exist, controls such
as proper respiratory protection or ventilation must be provided. Also, controls used to reduce atmospher-ic
contaminants to acceptable levels must be tested regularly. Where adverse atmospheric conditions may
exist or develop in an excavation, the employer also must provide and ensure that emergency rescue
equipment, ( e. g., breathing apparatus, a safety harness and line, basket stretcher, etc.) is readily avail-able.
This equipment must be attended when used.
When an employee enters bell- bottom pier holes and similar deep and confined footing excavations, the
employee must wear a harness with a lifeline. The lifeline must be securely attached to the harness and
must be separate from any line used to handle materials. Also, while the employee wearing the lifeline is
in the excavation, an observer must be present to ensure that the lifeline is working properly and to
maintain communication with the employee.
Access and Egress
Under the standard, the employer must provide safe access and egress to all excavations. According to
OSHA regulations, when employees are required to be in trench excavations 4 feet deep ( 1.22 meters) or
more, adequate means of exit, such as ladders, steps, ramps or other safe means of egress, must be pro-vided
and be within 25 feet ( 7.62 meters) of lateral travel. If structural ramps are used as a means of
access or egress, they must be designed by a competent person if used for employee access or egress, or a
competent person qualified in structural design if used by vehicles. Also, structural members used for
ramps or runways must be uniform in thickness and joined in a manner to prevent tripping or displace-ment.
28
5
Residential Contractors and the Excavations Standard
Residential builders and contractors face a unique set of circumstances when building homes. Both the
federal and North Carolina departments of labor have recognized that residential construction sites can
be very different from commercial sites as they relate to part of the OSHA Excavations Standard ( 29 CFR
1926.652). To address accepted residential building practices as they relate to the standard, both agencies
have adopted similar occupational safety and health enforcement policies for excavations on residential
sites.
Residential builders in North Carolina must follow the N. C. Department of Labor’s version of this poli-cy.
The department’s policy suspends 29 CFR 1926.652 for house foundation and basement excavations at
residential sites when all of the following conditions are present:
• The house foundation/ basement excavation is less than 71/ 2 feet in depth or is benched for at least 2
feet horizontally for every 5 feet or less of vertical height.
• The minimum horizontal width ( excavation face to formwork/ wall) at the bottom of the excavation is
as wide as practicable but not less than 2 feet.
• There is no water, surface tension cracks or other environmental conditions present that reduce the
stability of the excavations.
• There is no heavy equipment operating in the vicinity that causes vibration to the excavation while
employees are in the excavation. All soil, equipment and material surcharge loads are no closer in
distance to the top edge of the excavation than the excavation is deep; however, when front- end load-ers
are used to dig the excavations, the soil surcharge load must be placed as far back from the edge
of the excavation as possible, but never closer than 2 feet.
• Work crews in the excavation are the minimum number needed to perform the work.
• The work has been planned and is carried out in a manner to minimize the time employees are in
the excavation.
29
6
Worker Training and Jobsite Safety
Suggested Company Policy
One of the most important responsibilities of field and office management is planning for safety. Most
on- the- job problems and accidents directly result from improper planning. Correcting mistakes in shoring
and/ or sloping after work has begun slows down the operation, adds to the cost and increases the possibil-ity
of an excavation failure.
Contractors should develop safety checklists to make certain that there is enough information about
the jobsite and that all needed items, such as safety equipment, are on hand.
To help ensure safety in trenching and excavations, these specific conditions should be taken into
account:
• Soil types and layers
• Traffic
• Nearness of structures and their condition
• Surface and ground water conditions
• The water table elevation
• Overhead and underground utilities
• Weather
These and other conditions can be determined by jobsite studies, observations, test borings and consul-tations
with local officials and utility companies. Underground installations— sewer, telephone, water,
fuel and electric lines— that may be encountered in the excavation must be located before starting the job.
If underground installations are uncovered, OSHA regulations require that they be properly supported.
The contractor must contact the utility companies involved and inform them of the proposed work before
starting the trench or excavation.
Companies should establish a safety and health program. This important aspect of the work is put into
place before the job starts. Field and office personnel should become familiar with the company policies
and guidelines outlined in their company safety program. The program is often put in writing to solidify
and communicate the company’s position with regard to jobsite safety. Additionally, OSHA has specific
training requirements for all employees who are required to enter confined or enclosed spaces.
Cooperation from supervisors, employee groups and individual employees is necessary to ensure that
safety policies are implemented effectively. In addition, each supervisor must understand his or her
degree of responsibility for providing a safe working environment.
The cooperation of all employees requires their recognition of safety hazards and the necessary safety
precautions. Employees should be trained in the following areas:
1. Hazards associated with trenching and excavating
2. Soil identification
3. Safe slopes for different soil types and conditions
4. Proper installation and shoring
5. Stress patterns on trench walls from soil and spoil, equipment, and vibration caused by equipment
and traffic
6. Effects of adjacent buried utilities, building foundations and lengthy exposure to the elements on
trench side walls and other excavations
30
7. Effects on trench and excavation conditions from severe weather, such as excess water, freezing tem-peratures,
unexpected heat or prolonged drying
8. Recognition of buried drums, containers, tanks and wells
Employees should be trained to follow proper procedures to involve the electrical power company,
health department, and other agencies upon the discovery of unforeseen objects such as wells, sewage
disposal systems, cemeteries, and historic or architectural artifacts.
An example of a company safety and health program in action is a written policy that ensures that all
employees in all excavations will be protected from cave- ins. All company personnel will be expected to do
their part to enforce this policy.
On- the- Job Follow- Up
Once the job gets underway, each employer should keep itself informed of the safety aspects of the pro-ject
as well as the progress of the work. This is called on- the- job follow- up and involves a series of inspec-tions
to detect hazards and correct jobsite situations before cave- ins or other accidents occur. When man-agement
requires daily reports, acts on the reports and makes personal visits to the jobsite, it may feel
more confident that everyone is meeting job safety responsibilities.
Special Health and Safety Considerations
Competent person. The designated competent person should have and be able to demonstrate the fol-lowing:
Training, experience and knowledge of:
- soil analysis;
- use of protective systems; and
- requirements of 29 CFR Part 1926 Subpart P.
Ability to detect:
- conditions that could result in cave- ins;
- failures in protective systems;
- hazardous atmospheres; and
- other hazards including those associated with confined spaces.
Authority to take prompt corrective measures to eliminate existing and predictable hazards and to stop
work when required.
Surface crossing of trenches. Surface crossing of trenches should be discouraged; however, if
trenches must be crossed, such crossings are permitted only under the following conditions:
Vehicle crossings must be designed by and installed under the supervision of a registered professional
engineer.
Walkways or bridges must be provided for foot traffic. These structures must:
- have a safety factor of 4;
- have a minimum clear width of 20 inches ( 0.51 meters);
- be fitted with standard rails; and
- extend a minimum of 24 inches ( 0.61 meters) past the surface edge of the trench.
Ingress and egress. Access to and exit from the trench require the following conditions:
Trenches 4 feet or more in depth should be provided with a fixed means of egress. Spacing between lad-ders
or other means of egress must be such that a worker will not have to travel more than 25 feet lat-erally
to the nearest means of egress. Ladders must be secured and extend a minimum of 36 inches ( 0.9
meters) above the landing. Metal ladders should be used with caution, particularly when electric utili-ties
are present.
31
Exposure to vehicles. Procedures to protect employees from being injured or killed by vehicle traffic
include:
Providing employees with and requiring them to wear warning vests or other suitable garments
marked with or made of reflectorized or high- visibility materials. Requiring a designated, trained flag-person
along with signs, signals and barricades when necessary.
Exposure to falling loads. Employees must be protected from loads or objects falling from lifting or
digging equipment. Procedures designed to ensure their protection include:
Employees are not permitted to work under raised loads. Employees are required to stand away from
equipment that is being loaded or unloaded. Equipment operators or truck drivers may stay in their
equipment during loading and unloading if the equipment is properly equipped with a cab shield or
adequate canopy.
Warning systems for mobile equipment. The following steps should be taken to prevent vehicles
from accidentally falling into the trench:
Barricades must be installed where necessary. Hand or mechanical signals must be used as required.
Stop logs must be installed if there is a danger of vehicles falling into the trench. Soil should be graded
away from the excavation; this will assist in vehicle control and channeling of run- off water.
Hazardous atmospheres and confined spaces. Employees must not be permitted to work in haz-ardous
and/ or toxic atmospheres. Such atmospheres include those with:
Less than 19.5 percent or more than 23.5 percent oxygen; combustible gas concentration greater than
20 percent of the lower flammable limit; and concentrations of hazardous substances that exceed those
specified in the Threshold Limit Values for Airborne Contaminants established by the American
Conference of Governmental Industrial Hygienists.
All operations involving such atmospheres must be conducted in accordance with OSHA requirements
for occupational health and environmental controls ( see Subpart D of 29 CPR 1926) for personal protec-tive
equipment and for lifesaving equipment ( see Subpart E, 29 CFR 1926). Engineering controls ( e. g.,
ventilation) and respiratory protection may be required.
When testing for atmospheric contaminants, the following should be considered:
• Testing should be conducted before employees enter the trench and should be done regularly to
ensure that the trench remains safe. The frequency of testing should be increased if equipment is
operating in the trench. Testing frequency should also be increased if welding, cutting or burning is
done in the trench.
• Employees required to wear respiratory protection must be trained, fit- tested and enrolled in a res-piratory
protection program. Some trenches qualify as confined spaces. When this occurs, compli-ance
with the Confined Space Standard is also required.
Emergency rescue equipment. Emergency rescue equipment is required when a hazardous atmos-phere
exists or can reasonably be expected to exist. Requirements are as follows:
• Respirators must be of the type suitable for the exposure. Employees must be trained in their use,
and a respirator program must be instituted.
• Attended ( at all times) lifelines must be provided when employees enter bell- bottom pier holes, deep
confined spaces or other similar hazards.
• Employees who enter confined spaces must be trained.
Standing water and water accumulation. Methods for controlling standing water and water accu-mulation
must be provided and should consist of the following if employees are permitted to work in the
excavation:
• Use of special support or shield systems approved by a registered professional engineer
• Water removal equipment, i. e., well pointing, used and monitored by a competent person
32
• Safety harnesses and lifelines used in conformance with 29 CFR 1926.104
• Surface water diverted away from the trench
• Employees removed from the trench during rainstorms
• Trenches carefully inspected by a competent person after each rain and before employees are permit-ted
to re- enter the trench.
Inspections. A competent person must inspect the trench and document any findings. The following
guide specifies the frequency and conditions requiring inspections:
• Daily and before the start of each shift
• As dictated by the work being done in the trench
• After every rainstorm
• After other events that could increase hazards, e. g. snowstorm, windstorm, thaw, earthquake, etc.
• When fissures, tension cracks, sloughing, undercutting, water seepage, bulging at the bottom or
other similar conditions occur
• When there is a change in the size, location or placement of the spoil pile
• When there is any indication of change or movement in adjacent structures.
33
7
Safety Checklist*
FOR A SAFE JOBSITE TRENCHING OPERATION, THE ANSWER TO EACH OF THE FOLLOW-ING
SHOULD BE YES.
YES NO
General Protection Requirements
____ ____ Are all walkways, runways and sidewalks clear of excavated material or other obstructions?
____ ____ Are all undermined sidewalks shored to carry a minimum live load of 125 pounds per square
foot?
____ ____ Are planks laid parallel to the length of walk and fastened together against displacement if
used for raised walkways, runways or sidewalks?
____ ____ Are planks uniform in thickness?
____ ____ Are cleats on ramps provided when ramps are used in lieu of steps?
____ ____ Are all employees engaged in excavation, trenching and shoring provided with the appropriate
training and personal protective equipment?
____ ____ Are employees exposed to vehicular traffic wearing warning vests marked with or made of
reflectorized or high visibility material?
____ ____ Are employees prohibited from being under suspended loads handled by lifting and other
heavy equipment?
____ ____ Are daily inspections made of excavations, trenching and shoring?
____ ____ If evidence of possible cave- ins or slides is apparent, does all work stop until the necessary
precautions have been taken to safeguard all the employees?
Specific Excavation Requirements
____ ____ Prior to opening an excavation and/ or trench, has an effort been made to determine if any
underground installations will be encountered?
____ ____ Did you contact utility companies before excavating?
____ ____ Did you remove all trees, boulders and other surface encumbrances before excavating was
begun?
____ ____ Are the walls and faces of all excavations in which employees are exposed to danger from mov-ing
ground guarded by a shoring system, ground sloping or some other equivalent means?
____ ____ Are excavations inspected by a competent person after every rainstorm or other hazard-increasing
occurrence to determine if additional protection against slides and cave- ins is nec-essary?
____ ____ Did you store or retain excavated or other materials at least 2 feet or more from the edges of
the excavation?
____ ____ Do sides, slopes and faces of all excavations meet accepted engineering requirements by tier-ing,
barricading, rock bolting, wire meshing or other equally effective means?
____ ____ When excavations are in excess of 20 feet in depth, was the support system designed by a reg-istered
professional engineer?
34
* Adapted from Checklist for Trenching and Shoring, Kentucky Occupational Safety and Health Administration
( please see Suggested Readings).
YES NO
____ ____ Are all materials used for sheeting, piling, cribbing, bracing, shoring and underpinning in
good serviceable condition?
____ ____ Is the timber used for this purpose free of large or loose knots?
____ ____ Are diversion ditches, dikes or other suitable means used to prevent surface water from enter-ing
an excavation and to provide adequate drainage to the area?
____ ____ When heavy equipment, material or objects are on a level above and near an excavation, are
the sides of the excavation sheet- piled, shored and braced?
____ ____ When mobile equipment is used or is adjacent to excavations, are stop logs or barricades
installed?
____ ____ Are adequate barriers provided at all remotely located excavations? Are all wells, pits, shafts,
etc., adequately covered?
____ ____ On completion of the operation, are all temporary wells, pits, shafts, etc., backfilled?
____ ____ If possible, are dust conditions kept to a minimum?
____ ____ In locations where oxygen deficiency or gaseous conditions are possible, is the air in the exca-vation
tested?
____ ____ Where employees and/ or equipment are required to cross over excavations, are walkways or
bridges with standard guardrails provided?
____ ____ Do ladders used in excavation operations meet the OSHA requirements, and are ladders suffi-cient
in number?
____ ____ Are ladders or other means of exiting the trench provided and located so as to require no more
than 25 feet of lateral travel by employees to use them?
Specific Trenching Requirements
____ ____ Are all banks more than 5 feet high shored or laid back to a stable slope?
____ ____ Are the sides of trenches or embankments in Type B soil 4 feet or more in depth shored, sheet-ed,
braced, benched, sloped or otherwise sufficiently supported?
____ ____ Are sides of trenches or embankments in Type A soil shored or otherwise supported when the
excavation is 4 feet deep?
____ ____ When excavations or trenches are located near backfills or where they are subjected to vibra-tions
from railroad or highway traffic or machinery operation, are precautions such as shoring
and bracing taken to prevent slides or cave- ins?
____ ____ Is the excavation braced and shored as soon as it is opened?
____ ____ Are all crossbraces or jacks placed in true horizontal position?
____ ____ Are the vertical braces properly spaced and secured to prevent sliding, falling or kickouts?
____ ____ Where trench boxes or shields are used, are they designed and constructed in a manner that
will provide protection for the employees?
____ ____ Do backfilling and removal of trench supports progress together from the bottom of the
trench?
____ ____ Are jacks or braces released slowly?
____ ____ In unstable soil, are ropes used to pull out jacks or braces from above, after employees are
safely away from the trench?
35
Definitions
Accepted engineering practices are procedures compatible with the standards of practice required
of a registered professional engineer.
Adjacent structure stability refers to the stability of the foundations of adjacent structures whose
location may create surcharges, changes in soil conditions or other disruptions that have the potential to
extend into the failure zone of the excavation or trench.
Competent person is an individual who is capable of identifying existing and predictable hazards or
working conditions that are hazardous, unsanitary or dangerous to employees, and who has authoriza-tion
to take prompt corrective measures to eliminate or control these hazards and conditions.
Confined space is a space that, by design and/ or configuration, has limited openings for entry and
exit, unfavorable natural ventilation, may contain or produce hazardous substances, and is not intended
for continuous employee occupancy.
Excavation. An excavation is any man- made cut, cavity, trench or depression in an earth surface that
is formed by earth removal. A trench is a narrow excavation ( in relation to its length) made below the
surface of the ground. In general, the depth of a trench is greater than its width, and the width ( mea-sured
at the bottom) is not greater than 15 feet ( 4.6 meters). If a form or other structure installed or con-structed
in an excavation reduces the distance between the form and the side of the excavation to 15 feet
( 4.6 meters) or less ( measured at the bottom of the excavation), the excavation is also considered to be a
trench.
Hazardous atmosphere is an atmosphere that by reason of being explosive, flammable, poisonous,
corrosive, oxidizing, irritating, oxygen- deficient, toxic or otherwise harmful may cause death, illness or
injury to persons exposed to it.
Ingress and egress mean “ entry” and “ exit,” respectively. In trenching and excavation operations,
they refer to the provision of safe means for employees to enter or exit an excavation or trench.
Protective system refers to a method of protecting employees from cave- ins, from material that could
fall or roll from an excavation face or into an excavation, and from the collapse of adjacent structures.
Protective systems include support systems, sloping and benching systems, shield systems, and other sys-tems
that provide the necessary protection.
Registered professional engineer is a person who is registered as a professional engineer in the
state where the work is to be performed. However, a professional engineer who is registered in any state
is deemed to be a registered professional engineer within the meaning of Subpart P when approving
designs for manufactured protective systems or tabulated data to be used in interstate commerce.
Support system refers to structures such as underpinning, bracing and shoring that provide support
to an adjacent structure or underground installation or to the sides of an excavation or trench.
Subsurface encumbrances include underground utilities, foundations, streams, water tables, trans-former
vaults and geological anomalies.
Surcharge means an excessive vertical load or weight caused by spoil, overburden, vehicles, equip-ment
or activities that may affect trench stability.
Tabulated data are tables and charts approved by a registered professional engineer and used to
design and construct a protective system.
Underground installations include, but are not limited to, utilities ( sewer, telephone, fuel, electric,
water and other product lines), tunnels, shafts, vaults, foundations and other underground fixtures or
equipment that may be encountered during excavation or trenching work.
Unconfined compressive strength is the load per unit area at which soil will fail in compression.
This measure can be determined by laboratory testing, or it can be estimated in the field using a pocket
penetrometer, by thumb penetration tests or by other methods.
36
Suggested Readings
General References
29 CFR 1926 § § 650– 652 and appendixes A– E
Brunst, Gregory J. “ Unique Shoring Equipment Increase Trench Safety.” Concrete Construction ( June
1986): 46.
Carson, Brinton A. General Excavation Methods. New York: F. W. Dodge Corp., 1961.
Checklist for Trenching and Shoring ( Revised). Kentucky Labor Cabinet, Occupational Safety and Health
Program, Division of Education and Training. April 1985.
“ Cut and Cover Construction on Unstable Slopes.” Journal of the Construction Division. Proceeding of
ASCE. Vol. 106, No. C04 ( December 1980): 585– 597.
Excavations. Occupational Safety and Health Administration. OSHA 2226.
Manual of Accident Prevention in Construction. Washington, D. C.: Associated General Contractors of
America, 1971.
Safety and Health in Excavation and Trenching Operations. U. S. Department of Labor, Occupational
Safety and Health Administration. 2091.
“ Trenching Safety for Concrete Contractors.” Concrete Construction ( June 1986): 556– 559.
Technical References
American Standard Safety Code for Building Construction. Institute of Architects. National Safety
Council. June 1944.
Durham, C. W. Foundation of Structures. New York: McGraw- Hill Book Company Inc., 1950.
“ Excavation Stabilized Using Rock Revetments.” Journal of Geotechnical Engineering. Vol. 109, No. 3
( March 1983): 424– 439.
“ Field Measurements of an Earth Support System.” Journal of the Geotechnical Engineering Division.
Proceeding of ASCE. Vol. 107, No. GT 12 ( December 1981): 1625– 1642.
Keen, B. The Physical Properties of Soil. New York: Longmans, Green, and Company Inc., 1931.
Krynine, D. P., and W. R. Judd. Principles of Engineering Geology and Geotechnics. New York: McGraw-
Hill Book Company Inc., 1957.
Nunnally, S. W. Construction Methods and Management. New Jersey: Simon and Schuster, 1987.
Peck, Hanson and Thornburn. Foundation Engineering. New York: John Wiley and Sons, 1974.
Peck, R. B., et al. Foundation Engineering. New York: John Wiley and Sons, 1974.
Peurifoy, Robert L., and Garold D. Oberlender. Estimating Construction Costs. New York: McGraw- Hill
Book Company Inc., 1989. 117– 122.
Swiger, F. W. “ Control of Ground Water in Excavations.” ASCE Journal Construction Division. Vol. 86, No.
C01: 41– 53.
Terzaghi, K., and R. B. Peck. Soil Mechanics in Engineering Practice. New York: John Wiley and Sons,
1948.
“ Trench Excavation Effects on Adjacent Buried Pipes: Finite Element Study.” Journal of Geotechnical
Engineering. Vol. 109, No. 11 ( November 1983): 1399– 1415.
U. S. Department of Labor, OSHA Technical Manual ( Section V: Chapter II), Excavations: Hazard
Recognition in Trenching and Shoring.
37
Werblin, D. A. “ Installation and Operation of Dewatering Systems.” ASCE Journal Soil Mech. 86 ( 1960):
47– 66.
Yokel, Tucker, Lymon, Reese. Soil Classification for Construction Practice in Shallow Trenching. U. S.
Department of Commerce, National Bureau of Standards.
38
The following industry guides are available from the N. C. Department of Labor’s Division of Occupational
Safety and Health:
1# 1. A Guide to Safety in Confined Spaces
1# 2. A Guide to Procedures of the Safety and Health Review Board of North Carolina
1# 3. A Guide to Machine Safeguarding
1# 4. A Guide to OSHA in North Carolina
1# 5. A Guide for Persons Employed in Cotton Dust Environments
1# 6. A Guide to Lead Exposure in the Construction Industry
1# 7. A Guide to Bloodborne Pathogens in the Workplace
1# 8. A Guide to Voluntary Training and Training Requirements in OSHA Standards
1# 9. A Guide to Ergonomics
# 10. A Guide to Farm Safety and Health
# 11. A Guide to Radio Frequency Hazards With Electric Detonators
# 12. A Guide to Forklift Operator Training
# 13. A Guide to the Safe Storage of Explosive Materials
# 14. A Guide to the OSHA Excavations Standard
# 15. A Guide to Developing and Maintaining an Effective Hearing Conservation Program
# 17. A Guide to Asbestos for Industry
# 18. A Guide to Electrical Safety
# 19. A Guide to Occupational Exposure to Wood, Wood Dust and Combustible Dust Hazards
# 20. A Guide to Crane Safety
# 21. A Guide to School Safety and Health
# 23. A Guide to Working With Electricity
# 25. A Guide to Personal Protective Equipment
# 26. A Guide to Manual Materials Handling and Back Safety
# 27. A Guide to the Control of Hazardous Energy ( Lockout/ Tagout)
# 28. A Guide to Eye Wash and Safety Shower Facilities
# 29. A Guide to Safety and Health in Feed and Grain Mills
# 30. A Guide to Working With Corrosive Substances
# 31. A Guide to Formaldehyde
# 32. A Guide to Fall Prevention in Industry
# 33. A Guide to Office Safety and Health
# 34. A Guide to Safety and Health in the Poultry Industry
# 35. A Guide to Preventing Heat Stress
# 36. A Guide to the Safe Use of Escalators and Elevators
# 37. A Guide to Boilers and Pressure Vessels
# 38. A Guide to Safe Scaffolding
# 39. A Guide to Safety in the Textile Industry
# 40. A Guide to Emergency Action Planning
# 41. A Guide to OSHA for Small Businesses in North Carolina
Occupational Safety and Health ( OSH)
Sources of Information
You may call 1- 800- NC- LABOR ( 1- 800- 625- 2267) to reach any division of the N. C. Department of Labor; or visit the
NCDOL home page on the World Wide Web, Internet Web site address: http:// www. nclabor. com.
N. C. Division of Occupational Safety and Health
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 3rd Floor)
Local Telephone: ( 919) 807- 2900 Fax: ( 919) 807- 2856
For information concerning education, training and interpretations of occupational safety and health standards contact:
Bureau of Education, Training and Technical Assistance
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 4th Floor)
Telephone: ( 919) 807- 2875 Fax: ( 919) 807- 2876
For information concerning occupational safety and health consultative services and safety awards programs contact:
Bureau of Consultative Services
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 3rd Floor)
Telephone: ( 919) 807- 2899 Fax: ( 919) 807- 2902
For information concerning migrant housing inspections and other related activities contact:
Agricultural Safety and Health Bureau
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 2nd Floor)
Telephone: ( 919) 807- 2923 Fax: ( 919) 807- 2924
For information concerning occupational safety and health compliance contact:
Safety and Health Compliance District Offices
Raleigh District Office ( 313 Chapanoke Road, Raleigh, NC 27603)
Telephone: ( 919) 779- 8570 Fax: ( 919) 662- 4709
Asheville District Office ( 204 Charlotte Highway, Suite B, Asheville, NC 28803- 8681)
Telephone: ( 828) 299- 8232 Fax: ( 828) 299- 8266
Charlotte District Office ( 901 Blairhill Road, Suite 200, Charlotte, NC 28217- 1578)
Telephone: ( 704) 665- 4341 Fax: ( 704) 665- 4342
Winston- Salem District Office ( 4964 University Parkway, Suite 202, Winston- Salem, NC 27106- 2800)
Telephone: ( 336) 776- 4420 Fax: ( 336) 776- 4422
Wilmington District Office ( 1200 N. 23rd St., Suite 205, Wilmington, NC 28405- 1824)
Telephone: ( 910) 251- 2678 Fax: ( 910) 251- 2654
*** To make an OSHA Complaint, OSH Complaint Desk: ( 919) 807- 2796***
For statistical information concerning program activities contact:
Planning, Statistics and Information Management
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 2nd Floor)
Telephone: ( 919) 807- 2950 Fax: ( 919) 807- 2951
For information about books, periodicals, vertical files, videos, films, audio/ slide sets and computer databases contact:
N. C. Department of Labor Library
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 5th Floor)
Telephone: ( 919) 807- 2848 Fax: ( 919) 807- 2849
N. C. Department of Labor ( Other than OSH)
1101 Mail Service Center
Raleigh, NC 27699- 1101
Telephone: ( 919) 733- 7166 Fax: ( 919) 733- 6197

Click tabs to swap between content that is broken into logical sections.

A Guide to the
OSHA Excavations Standard
Bobby R. Davis
Series Editor
Division of Occupational Safety and Health
N. C. Department of Labor
1101 Mail Service Center
Raleigh, NC 27699- 1101
Cherie K. Berry
Commissioner of Labor
N. C. Department of Labor
Occupational Safety and Health Program
Cherie K. Berry
Commissioner of Labor
OSHA State Plan Designee
Allen McNeely
Deputy Commissioner for Safety and Health
Kevin Beauregard
Assistant Deputy Commissioner for Safety and Health
Acknowledgments
This edition of A Guide to the OSHA Excavation Standard has been updated to include material as
prepared by the Construction Education and Research Institute, Department of Civil Engineering at
N. C. State University ( principal investigators were Paul P. McCain and David W. Johnston); U. S.
Department of Labor, Occupational Safety and Health Administration; U. S. Department of Health and
Human Services ( NIOSH); and OSHNC personnel. This guide is intended to be consistent with exist-ing
state and federal OSHA standards. Therefore, if the reader considers a statement to be inconsis-tent
with a standard, the OSHA standard should be followed.
To obtain additional copies of this book, or if you have questions about N. C. occupational safety and
health standards or rules, please contact:
N. C. Department of Labor
Bureau of Education, Training and Technical Assistance
1101 Mail Service Center
Raleigh, NC 27699- 1101
Phone: ( 919) 807- 2875 or 1- 800- NC- LABOR ( 1- 800- 625- 2267)
____________________
Additional sources of information are listed on the inside back cover of this book.
____________________
The projected cost of the OSHNC program for federal fiscal year 2000– 2001 is $ 14,152,395. Federal funding provides approximately 30 percent
($ 4,528,766) of this total.
Printed 02/ 02, 1M
Contents
Part Page
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1iiv
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1vii
1 Common Soil Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1ii1
2 Soil Types and Pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . ii18
3 Methods of Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii14
4 Installation and Removal of Protective Systems . . . . . . . . . . . . . . ii18
5 Residential Contractors and the Excavations Standard . . . . . . . . . ii29
6 Worker Training and Jobsite Safety . . . . . . . . . . . . . . . . . . . . . ii30
7 Safety Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii34
Suggested Readings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii37
iii
Foreword
There is no reason why anyone ever has to die in a trenching accident. Modern technology has provided
us with a variety of excellent shoring systems and trench shields. The OSHA excavations standard pro-vides
us with a set of clearly written and logical safety rules. Yet every year workers are killed and injured
by cave- ins. Employers and employees can expect N. C. Department of Labor safety inspectors to make the
most detailed, professional inspections possible whenever such accidents occur.
A Guide to the OSHA Excavations Standard examines the standard’s different sections, offering many
illustrations and a safety checklist to help explain how to excavate and work safely in a trench. It also dis-cusses
the new rule concerning excavating and residential construction.
In North Carolina, DOL inspectors enforce the federal Occupational Safety and Health Act through a
state plan approved by the U. S. Department of Labor. The N. C. Department of Labor’s Division of
Occupational Safety and Health offers many educational programs to the public and produces publica-tions,
including this guide, to help inform people about their rights and responsibilities regarding occupa-tional
safety and health.
When looking through this guide, please remember DOL’s mission is greater than just to enforce regu-lations.
An equally important goal is to help people find ways to create safe workplaces. This booklet, like
the other educational materials produced by the N. C. Department of Labor, can help.
Cherie K. Berry
Commissioner of Labor
v
Introduction
The Occupational Safety and Health Administration issued its first Excavation and Trenching
Standard in 1971 to protect workers from excavation hazards. Since then, OSHA has amended the stan-dard
several times to increase worker protection and to reduce the frequency and severity of excavation
accidents and injuries. Despite these efforts, excavation- related accidents resulting in injuries and fatali-ties
continue to occur.
OSHNC has developed this guide in effort to address requirements of the standard, as well as provide
information for equipment operators, workers and all others associated with trenching and excavating to
help recognize hazardous conditions that could result in injury or a fatality. This guide discusses soil com-position
in moderate detail to provide a general overview of the various properties associated with differ-ent
types of soil. A general understanding of the properties of soil is the first step in predicting the behav-ior
of soils in varying condition. Some of the most common types of soil conditions that lead to trench and
excavation failure are also discussed.
Proper trenching operations are necessary to protect the workers from soil collapse. The basic trench-ing
operations that help make a trench safe for workers are described and illustrated. The methods of
shoring installation are also discussed briefly. This document is not intended to be used as a step-by-
step guideline in the excavation process.
OSHA has completely updated the excavations standard, with focus on the existing standard to simpli-fy
many of the existing provisions, add and clarify definitions, eliminate duplicate provisions and ambigu-ous
language, and give employers added flexibility in providing protection for employees. In addition, the
standard provides several new appendices. Appendix A to 1926.652 provides a consistent method of soil
classification. Appendix B to 1926.652 provides sloping and benching requirements. Other appendixes
( appendixes C– F) provide pictorial examples of shoring and shielding devices, timber tables, hydraulic
shoring tables, and selection charts that provide a graphic summary of the requirements contained in the
standard. For more information on the details of proper installation, please refer to the OSHA standard
on excavation ( 29 CFR 1926 Subpart P, which includes 650– 652 and appendixes A– F) and to the
Suggested Readings in this guide.
Scope and Application
OSHA’s revised rule applies to all open excavations made in the earth’s surface, which includes trench-es.
According to the OSHA construction safety and health standards, 1926.650( b), a trench is referred to
as a narrow excavation made below the surface of the ground in which the depth is greater than the
width— the width not exceeding 15 feet ( 4.5 meters). An excavation is any man- made cut, cavity, trench
or depression in the earth’s surface formed by earth removal. This can include excavations for anything,
from cellars to highways. The standard does not apply to house foundation/ basement excavations ( includ-ing
those that become trenches by definition when formwork, foundations or walls are constructed) when
all of the following conditions are present:
• The house foundation/ basement excavation is less than 71/ 2 feet ( 2.5 meters) deep or is benched for at
least 2 feet ( 0.61 meters) horizontally for every 5 feet ( 1.52 meters) or less of vertical height;
• The minimum horizontal width ( excavation face to formwork/ wall) at the bottom of the excavation is
as wide as practicable but not less than 2 feet ( 0.61 meters);
• There is no water, surface tension cracks nor other environmental conditions present that reduce the
stability of the excavation;
• There is no heavy equipment operating in the vicinity that causes vibration to the excavation while
employees are in the excavation;
• All soil, equipment and material surcharge loads are no closer in distance to the top edge of the exca-vation
than the excavation is deep; however, when front end loaders are used to dig the excavations,
the soil surcharge load must be placed as far back from the edge of the excavation as possible, but
never closer than 2 feet ( 0.61 meters).
vii
• Work crews in the excavation are the minimum number needed to perform the work; and
• The work has been planned and is carried out in a manner to minimize the time employees are in
the excavation.
These conditions as applicable to residential construction are restated in another section of this guide,
Residential Contractors and the Excavations Standard.
The standard provides several options for designing trench protection measures. Under certain condi-tions,
the design of the trench protection measures must be prepared by a registered professional engi-neer.
Proper selection and installation of trench protection measures are very important. To comply with the
standard, the employer must have a competent person: “ one who is capable of identifying existing and
predictable hazards in the surroundings or working conditions which are unsanitary, hazardous, or dan-gerous
to employees, and who has authorization to take prompt corrective measures to eliminate them.”
This competent person has critical inspection responsibilities regarding excavations. This person must
inspect every excavation and protective system under his or her care daily, including areas adjacent to
any excavation. Additional inspections must be conducted before starting work and as needed through a
shift, for example, when any hazard- increasing occurrence ( such as a rainstorm) takes place. When the
competent person finds any evidence of a situation that could result in a cave- in, protective system failure
or any other hazardous condition, employees are to be immediately removed from the danger area until
the problem is fixed.
Many companies have established a written policy that outlines specific safe trenching practices in
detail. Such a policy should ensure adequate support for the trench and frequent inspections of the exca-vation
site to detect any change in the soil conditions. When this type of policy is enforced, all employees
understand their responsibilities, which helps to avoid unsafe practices.
This document is not intended to be a guideline for compliance with all pertinent OSHA
regulations but rather an overview of safe practices in trenching operations. Though the
guide is not intended to be inconsistent with OSHA standards, if an area is considered by the
reader to be inconsistent, the OSHA standard should be followed.
viii
1
Common Soil Problems
This guide highlights the requirements in the updated standard for excavation and trenching opera-tions,
provides methods for protecting employees against cave- ins, and describes safe work practices for
employees. A necessary first step in planning the approach to any trenching or other excavation project is
to understand what could go wrong. This understanding can help avoid many of the problems associated
with excavation.
The terms soil and earth are commonly referred to in the excavation process to describe the naturally
occurring materials uncovered on a project. Soil conditions vary from one site to the next. Soil may be
loose or partially cemented, organic or inorganic. However, most soils can be referred to as a mixture or
an accumulation of mineral grains that are not cemented together. An exception is hard rock, which
remains firm after exposure to the elements.
Soil failure is defined as the collapse of part or all of an excavation wall. The most common soil failure
is typically described as an unexpected settlement, or cave- in, of an excavation. Soil sliding is the most
common factor leading to soil failure.
Proper planning and supervision can avoid the unsafe working conditions caused by soil sliding. Unless
such safety precautions have been implemented, sliding soil failure can occur in all types of excavations
( including sloped trenches and excavations with braced trench boxes). See figure 1.
Overview: Soil Mechanics
A number of stresses and deformations can occur in an open cut or trench. For example, increases or
decreases in moisture content can adversely affect the stability of a trench or excavation. The following
diagrams show some of the more frequently identified causes of trench failure.
Tension cracks. Tension cracks usually form at a horizontal distance of one- half to three- quarters times
the depth of the trench, measured from the top of the vertical face of the trench. See figure 2 for addition-al
details.
Figure 1
Sliding Failure
1
Tension Crack
Failed Soil Mass
Figure 2
Tension Crack
Sliding or sluffing may occur as a result of tension cracks, as illustrated in figure 3.
Figure 3
Sliding
Toppling. In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench’s
vertical face shears along the tension crack line and topples into the excavation. See figure 4.
Figure 4
Toppling
2
Tension
Crack
H
.5 to .75 H
Sliding
Toppling
Subsidence and Bulging. An unsupported excavation can create an unbalanced stress in the soil,
which, in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncor-rected,
this condition can cause face failure and entrapment of workers in the trench. See figure 5.
Figure 5
Subsidence and Bulging
Heaving or Squeezing. Bottom heaving or squeezing is caused by the downward pressure created by the
weight of adjoining soil. This pressure causes a bulge in the bottom of the cut, as illustrated in figure 6.
Heaving and squeezing can occur even when shoring or shielding has been properly installed.
Figure 6
Heaving or Squeezing
3
Bulge
Subsidence
Heave Soil
Weight
Boiling is evidenced by an upward water flow into the bottom of the cut. A high water table is one of
the causes of boiling. Boiling produces a “ quick” condition in the bottom of the cut and can occur even
when shoring or trench boxes are used. See figure 7.
Figure 7
Boiling
Unit Weight of Soils refers to the weight of one unit of a particular soil. The weight of soil varies with
type and moisture content. One cubic foot of soil can weigh from 110 pounds to 140 pounds or more, and
one cubic meter ( 35.3 cubic feet) of soil can weigh more than 3,000 pounds.
A safe slope can be defined as the maximum angle of the edge wall or bank of an excavation at which
sliding will not occur. The unique mixtures of the different types of soil ( sand, clay, silt and rock) necessi-tate
different safe slopes from one excavation site to the next.
There are other complicating factors that can result in sliding soil failures. During an excavation, visi-bly
different layers of soil maybe uncovered. Each of those layers may call for different safe slopes. It is
essential to plan your excavation around the most gradual ( rather than steepest) safe slope for all of the
different soil types and layers encountered during the excavation.
Another complicating factor is that soil composition mixtures may vary significantly from one area of
the project to another. During an excavation, as the soil composition changes, the safe slope for trench
wall excavation also changes. Thus, across an excavation site, the slope of the bank may need to be differ-ent
to provide a safe working environment.
Sliding and other modes of failure can also occur in soils that are not densely compacted. For example,
a trench that is made close to a previously dug trench is very unstable. If uncompacted soil is discovered,
the normal safe slope for dense soil will not be enough to prevent sliding. Bracing or further sloping may
be necessary.
If cracks are observed in rocky types of soil, sliding has already occurred. These cracks should signal
that a more gradual slope for excavation is needed because the rocky soil is very susceptible to slides and
other types of failure.
Excavations that have been stable for long periods are also subject to sliding types of failure. After pro-longed
exposure to the elements, the moisture content in the soil may increase. This increase in moisture
may be due to various causes, such as rainfall or a broken water line. The extra soil moisture tends to
speed up sliding soil failures.
Determining the correct safe slope can be quite difficult for certain types of soil. The OSHA standard
has developed a simple method of determining safe excavation bank slopes for different soil types. This
new method will be discussed in more detail in a later section of this document.
4
Water
Table
Boiling
Soil failure can occur for any number of reasons. Factors that increase the chances of soil failure are:
1. excessive vibration
2. surface encumbrances
3. weather conditions
Cave- ins and Protective Support Systems
Excavation workers are exposed to many hazards, but the chief hazard is danger of cave- ins. OSHA
requires that in all excavations employees exposed to potential cave- ins must be protected by sloping or
benching the sides of the excavation, by supporting the sides of the excavation, or by placing a shield
between the side of the excavation and the work area. Designing a protective system can be complex
because of the number of factors involved- soil classification, depth of cut, water content of soil, changes
due to weather and climate, or other operations in the vicinity. The standard, however, provides several
different methods and approaches ( four for sloping and four for shoring, including the use of shields)* for
designing protective systems that can be used to provide the required level of protection against cave- ins.
One method of ensuring the safety and health of workers in an excavation is to slope the sides to an angle
not steeper than one and one- half horizontal to one vertical ( 34 degrees measured from the horizontal).
These slopes must be excavated to form configurations that are in accordance with those for Type C soil
found in appendix B of the standard. A slope of this gradation or less is considered safe for any type of
soil. ( See figure 8).
Figure 8
Type C Soil
Simple Slope Excavation
All simple slope excavations 20 feet ( 6.11 meters) or less in depth must have a maximum allowable
slope of 1.5: 1. A second design method, which can be applied for both sloping and shoring, involves using
tabulated data, such as tables and charts, approved by a registered professional engineer. These data
must be in writing and must include sufficient explanatory information to enable the user to make a
selection, including the criteria for determining the selection and the limits on the use of the data. At
least one copy of the information, including the identity of the registered professional engineer who
approved the data, must be kept at the worksite during construction of the protective system. Upon com-pletion
of the system, the data may be stored away from the jobsite, but a copy must be made available
upon request to OSHNC. Contractors may also use a trench box or shield that is either designed or
approved by a registered professional engineer or is based on tabulated data prepared or approved by a
registered professional engineer. Timber, aluminum or other suitable materials may also be used. OSHA
standards permit the use of a trench shield ( also known as a welder’s hut) as long as the protection it pro-vides
is equal to or greater than the protection that would be provided by the appropriate shoring system.
( See figure 9.)
5
* See appendix F to the standard for a complete overview of all options.
20' Maximum
11/ 2
1
Figure 9
Trench Shield
The standard does not require the installation and use of a protective system when an excavation is
made entirely in stable rock or is less than 5 feet deep ( 1.52 meters) and a competent person has exam-ined
the ground and found no indication of a potential cave- in.
Vibrations
Any large, heavy movement near an excavation results in vibration of the surrounding soils. This
movement can result in soil failure. Moving machinery, nearby traffic, pile driving and blasting all cause
vibration in surrounding soils.
Vibration- related soil failures can occur in all types of soil. However, certain types of soils are more sus-ceptible
to vibration failures than others. For example, sandy soils tolerate less vibration than clay soils.
Since actual soil conditions may be a mixture of more than one soil type, it is better to play it safe when
planning the slope of an excavation. Figure 10 shows typical situations where vibrations can result in soil
failure.
Figure 10
Two Examples of Vibration Failures
6
Strut
Knife Edge
Sidewall
Soil affected by the movement
of the crane and susceptible to
sliding
Soil affected by the movement
of the truck and susceptible to
sliding
Surface Encumbrances
Heavy loads such as large equipment, heavy materials or large spoil piles can be too heavy for the soil
to support, resulting in a cave- in. These loads are referred to as surface encumbrances. They pose differ-ent
types of dangers ( see figure 3). For example, large spoil piles may hide tension cracks that would oth-erwise
signal that a sliding soil failure may occur.
Figure 11
Surface Encumbrances
Existing site features such as buildings, curbs, trees, utility poles and other structures adjoining the
excavation area may be considered as types of surface encumbrances. These extra loads can place more
stress on the sides of an excavation than the walls can safely carry. Shoring, bracing, underpinning or
some combination of safety measures should be provided, as necessary, to protect workers and to prevent
movement of the soil beneath the adjacent load.
In cases where the space is limited and heavy loads must be located near an excavation, the trench
walls must be braced or shored as needed to safely support this extra weight.
Weather Conditions
Weather is an important factor in determining soil conditions. More importantly, changing weather
conditions may signal a change in the pressures exerted by the soil on the side walls of a trench.
Excess water from rain or melting snow interacts with the soil, increasing the pressure on the excava-tion
and shoring system. For instance, a rainstorm can turn a stable trench wall that required only light
bracing into a mass of loose soil that requires heavy bracing.
Freezing usually indicates a rather stable ground condition, unless the frost line is exceeded during exca-vation.
The frost line phenomenon is depicted in figure 12. If you excavate or shore frozen ground, be aware
that another potential problem exists— thawing. A sudden thaw can be as dangerous as a rainstorm.
Figure 12
Region of Soil Freezing
Excessively dry conditions can also be dangerous. As moisture content decreases, some dry soils lose
their ability to stick together. This lack of cohesion may result in a sliding type of soil failure. In many of
the situations described above, dewatering or extra shoring may be required as necessary to ensure the
safety of your workers. See figures 16 and 17 in part 2 for more information about dewatering.
7
Tension Crack
These loads not only create
forces too heavy for the soil
to carry, but also cover up
and signs of failure.
=
Ground level
Water level
Frost line
The water above the frost line can freeze
The water below the frost line does not freeze
2
Soil Types and Pressures
Soil Characteristics
The OSHA excavations standard recognizes and allows a variety of soil classification systems under
certain conditions. A special simple soil classification system used by OSHA for excavation planning and
protection is included in the standard. If that classification system is strictly followed, trench protection
systems can be designed for many situations without the approval of a registered professional engineer.
In the soil classification system used by OSHA, the terms used to identify soil types are drawn largely
from another system, commonly used for construction in North Carolina, called the United Soil
Classification System. Both systems are based upon the engineering properties of soils and are concise
and easily associated with actual soil behavior. The OSHA system can be applied in the laboratory or the
field. The terms used for classification are based upon the soil particles, including the quantity of the var-ious
sizes of soil particles and the qualities or characteristics of the very fine grains. The principal types
of soil may be divided into two general classes according to grain size. Coarse- grained soils are gravel and
sand. Fine- grained soils are silt and clay.
The composition or texture of a soil is a critical factor in its stability. The more cohesive the soil parti-cles,
the more the entire soil mass tends to stick together rather than crumble. However, it is important
to remember the time element involved in cuts. If an excavated cut is to be left open for long periods of
time, cohesive forces may not withstand exposure to weather conditions. When fresh fill dirt is not prop-erly
compacted, subsequent excavations in the same area result in almost no cohesion properties; thus, a
greater width may be required to maintain a stable slope.
The soil found at a site is usually a mixture of one or more of the basic types listed below. From the
amounts of each soil type blended together to form the actual soil conditions, descriptive soil terms are
combined in the order of lowest content to highest content. For example, soil classified as “ silty clay” is a
mixture of mostly clay with noticeable but lesser amounts of silt. The single term “ loam” is used to
describe a mixture of clay, sand and silt.
The types of soil found most often include:
Soil Type Characteristics of Soil Type
Clay Clay is a very, very fine- grained soil. In general, water moves very slowly through clay.
Large amounts of rainfall may pond on the surface and evaporate before being absorbed.
Sand Sand is a granular soil. The shape of individual grains may be round or angular. Sandy
soils tend to have large pores, allowing faster groundwater absorption. In most situations,
sandy soil is the easiest to compact with vibration.
Silt Silt has properties intermediate between fine sand and clay. Silt is the most sensitive to
changes in soil moisture content. Silt tends to crumble with drying.
Determination of Soil Type
By grouping different types of soils ( described above) according to requirements for safe excavation, the
excavation standard has defined four soil classifications ( provided below). For a detailed explanation of
OSHA classification system, please see appendix A of the excavation standard. OSHA categorizes soil and
rock deposits into four types, A through D, as follows:
Stable Rock is natural solid mineral matter that can be excavated with vertical sides and remain
intact while exposed. It is usually identified by a rock name such as granite or sandstone. Determining
whether a deposit is of this type may be difficult unless it is known whether cracks exist and whether or
not the cracks run into or away from the excavation.
8
Type A soils are cohesive soils with an unconfined compressive strength of 1.5 tons per square foot
( 144 kPa) or greater. Examples of Type A cohesive soils are clay, silty clay, sandy clay, clay loam and, in
some cases, silty clay loam and sandy clay loam. ( No soil is Type A if it is fissured, is subject to vibration
of any type, has previously been disturbed, is part of a sloped, layered system where the layers dip into
the excavation on a slope of four horizontal to one vertical or greater, or has seeping water.)
Type B soils are cohesive soils with an unconfined compressive strength greater than 0.5 tons per
square foot ( 48 kPa) but less than 1.5 ( 144 kPa). Examples of Type B soils are angular gravel; silt; silt
loam; previously disturbed soils unless otherwise classified as Type C; soils that meet the unconfined
compressive strength or cementation requirements of Type A soils but are fissured or subject to vibration;
dry unstable rock; and layered systems sloping into the trench at a slope less than four horizontal to one
vertical ( only if the material would be classified as a Type B soil).
Type C soils are cohesive soils with an unconfined compressive strength of 0.5 tons per square foot ( 48
kPa) or less. Type C soils include granular soils such as gravel, sand and loamy sand, submerged soil, soil
from which water is freely seeping, and submerged rock that is not stable. Also included in this classifica-tion
is material in a sloped, layered system where the layers dip into the excavation or have a slope of
four horizontal to one vertical or greater.
Layered geological strata. Where soils are configured in layers, i. e., where a layered geologic struc-ture
exists, the soil must be classified on the basis of the soil classification of the weakest soil layer. Each
layer may be classified individually if a more stable layer lies below a less stable layer, i. e., where a Type
C soil rests on top of stable rock.
The standard also contains other important criteria that must be examined to classify soils properly.
Important: 1. The laboratory testing process and compressive strength calculations must be conducted
under the direction of a registered professional engineer. The OSHA standard requires that the excava-tion
protection system be designed by a registered professional engineer when the depth of the excavation
exceeds 20 feet or where unusual site conditions exist. 2. The manual field testing alternative permitted
under the standard does not require the approval of a registered professional engineer under certain spe-cific
conditions. However, at least one visual test and one manual test are required to classify soil accord-ing
to the OSHA method. The specific manual and visual field tests are listed and described in the stan-dard.
Test Equipment and Methods for Evaluating Soil Type
Many kinds of equipment and methods are used to determine the type of soil prevailing in an area, as
described below.
Pocket Penetrometer. Penetrometers are direct- reading, spring- operated instruments used to deter-mine
the unconfined compressive strength of saturated cohesive soils. Once pushed into the soil, an indi-cator
sleeve displays the reading. The instrument is calibrated in either tons per square foot or kilograms
per square centimeter. However, penetrometers have error rates in the range of ± 20– 40 percent.
1. Shearvane ( Torvane). To determine the unconfined compressive strength of the soil with a shear-vane,
the blades of the vane are pressed into a level section of undisturbed soil, and the torsional
knob is slowly turned until soil failure occurs. The direct instrument reading must be multiplied by
2 to provide results in tons per square foot or kilograms per square centimeter.
2. Thumb Penetration Test. The thumb penetration procedure involves an attempt to press the thumb
firmly into the soil in question. If the thumb makes an indentation in the soil only with great diffi-culty,
the soil is probably Type A. If the thumb penetrates no further than the length of the thumb
nail, it is probably Type B soil, and if the thumb penetrates the full length of the thumb, it is Type C
soil. The thumb test is subjective and is therefore the least accurate of the three methods.
3. Dry Strength Test. Dry soil that crumbles freely or with moderate pressure into individual grains is
granular. Dry soil that falls into clumps that subsequently break into smaller clumps ( and the small-er
clumps can be broken only with difficulty) is probably clay in combination with gravel, sand or silt.
9
If the soil breaks into clumps that do not break into smaller clumps ( and the soil can be broken only
with difficulty), the soil is considered unfissured unless there is visual indication of fissuring.
Plasticity or Wet Thread Test. This test is conducted by molding a moist sample of the soil into a ball
and attempting to roll it into a thin thread approximately 1/ 8 inch ( 3 millimeters) in diameter ( thick) by 2
inches ( 50 millimeters) in length. The soil sample is held by one end. If the sample does not break or tear,
the soil is considered cohesive.
Visual Test. A visual test is a qualitative evaluation of conditions around the site. In a visual test, the
entire excavation site is observed, including the soil adjacent to the site and the soil being excavated. If
the soil remains in clumps, it is cohesive; if it appears to be coarse- grained sand or gravel, it is considered
granular. The evaluator also checks for any signs of vibration.
During a visual test, the evaluator should check for crack- line openings along the failure zone that
would indicate tension cracks, look for existing utilities that indicate that the soil has previously been
disturbed, and observe the open side of the excavation for indications of layered geologic structuring.
The evaluator should also look for signs of bulging, boiling or sluffing, as well as for signs of surface
water seeping from the sides of the excavation or from the water table. If there is standing water in the
cut, the evaluator should check for “ quick” conditions. In addition, the area adjacent to the excavation
should be checked for signs of foundations or other intrusions into the failure zone, and the evaluator
should check for surcharging and the spoil distance from the edge of the excavation.
Earth Pressures
An appreciation of the need for shoring can be based on an understanding of earth pressures. The
amount of earth pressure exerted upon the side wall of any excavation depends upon the weight and
depth of the soil that it supports.
Earth pressure distributions vary with the type of soil, depth of excavation and moisture conditions.
Example distributions are shown in figure 13.
The center of the earth pressures is normally found between one- half and two- thirds of the depth of a
simple excavation. However, additional earth pressures result from surface encumbrances and differ-ences
in soil layer cohesiveness. All of these factors influence the actual center of pressure at an excava-tion
site.
Figure 13
Pressure Distributions on the Side Walls of an Excavation
10
Sand and Granular
Soils
Soft Clay
Trench Wall
Pressure
Stiff Clay
As a general rule of thumb, the center of pressure will be at a lower depth when cohesion is poor ( as in
fresh fill dirt, water- bearing sand or loose ground). Where cohesion is high, the center of pressure will be
higher ( as in good compact soil). The location of the center of pressure can change after a cut is made
unless support is provided to prevent earth movement.
Regardless of the soil type, as the depth of the trench increases, the magnitude of pressures on the full
height of the excavation also increases. The presence of ground water adds hydrostatic pressure against
the walls of the trench as shown in figure 14.
Figure 14
Water Pressure
Effects of Water and Remedies
The natural water table can cause many types of problems. For example, trenches excavated below the
natural water table in sandy soils and soft clay are highly susceptible to heaving, as illustrated in figure
15. Heaving is the seepage of water at the bottom of the trench causing the soil to be pushed upward.
This heaving is a signal that a failure may occur.
Figure 15
Heaving
11
Ground water
table
Water
Pressure
Added effect of water pressure
Water Table
Dry Soil
Granular Saturated Soil
Heave
Granular
Soil
Water Movement
Wet conditions at the bottom of a trench may present another problem. If the bottom of the trench
begins to puff and bubble and the earth rises, a quicksand condition is occurring. This is also a signal
that a failure may occur.
If heaving or quicksand conditions are expected, dewatering should be considered prior to beginning an
excavation. Dewatering drastically reduces the presence of water and the additional pressure it causes.
Without dewatering, heavier timbers would be needed to support the extra pressures caused by the
water. The two most frequently used dewatering systems are well- points and sump pumps.
The well- point system ( illustrated in figure 16) is a very popular method of dewatering. Located on a
line 2 or 3 feet, or further, behind the sheeting, well- points are inserted to the depth of the excavation
with spacing between the well- points varying from 3 to 8 feet.
Figure 16
Well- Point
Well- points are pipes with a point at the lower end and a screen or filter over perforations along 3 or 4
feet of the lower ends of the pipes. There are two types of well- points:
those driven with a maul
those that are jetted in
The selection of the size of the well- points and the required spacing are based upon site conditions and
the type of excavation to be accomplished.
Above the ground, well- point pipes are connected by piping to a high- capacity pump. Pumping keeps
the water level below the bottom of the excavation so that only a moist soil condition will be encountered
within the excavation.
The well- point system should have a capacity sufficient to remove any inflow of water as quickly as it
occurs. The depth limit of this method’s practical effectiveness is approximately 15– 20 feet, although the
theoretical limit is just under 34 feet since the method depends upon pumping suction. Greater depths
can be achieved by arranging well- points into two or more vertical stages, or by deep- well pumping, that
is, locating the pump at a lower elevation.
Dewatering does not permit any substantial excavation without providing ground support. Although
the dewatered soil will usually be firmer than it was before dewatering, working conditions may still be
unsafe. Shoring, or bank walls at a safe slope, should be used in dewatered ground in the same manner
as in any other excavation.
12
Header
Sand and
gravel filter
Inflow pipe
covered with
filtering screen
Head
Jetting Holes
Lowered water table
Continuous
Sheeting
Ground
Trench
braces
Riser
The second common type of dewatering system is the sump pump as depicted in figure 17.
Figure 17
Sump Pump
Sump pumping, as contrasted with well- pointing, has several advantages. Sump pumps:
can be installed quickly by inexperienced labor
require less space and cause less interference on the site
can be added or removed easily to meet required pumping capacity
can be started by simply switching on the power supply because no balancing or turning is required
do not freeze in cold weather because of the fast, high volume flow of water
may be removed from one sump and used elsewhere if needed
usually cost less than well- points
13
Discharge pipe to a distant point
Pump
Suction pipe
Wire mesh
Coarse sand and
gravel filter
Bracing
Lowered water table
Sides supported by
sheeting until filter and
bracing are installed
3
Methods of Protection
Several methods of protecting workers in trench excavations are available. Some methods are pre-ferred
over others depending on the site specifics and circumstances. The various protection methods
available include:
1. sloping and benching
2. shoring ( spaced sheeting, close sheeting)
3. trench shield
4. other occasionally used systems
Each of these techniques is introduced briefly below and will be discussed in more detail in the pages
that follow.
Materials and Equipment
The employer is responsible for the safe condition of materials and equipment used for protective sys-tems.
Defective and damaged materials and equipment can result in the failure of a protective system
and cause excavation hazards. To avoid possible failure of a protective system, the employer must ensure
that:
1. materials and equipment are free from damage or defects,
2. manufactured materials and equipment are used and maintained in a manner consistent with the
recommendations of the manufacturer and in a way that will prevent employee exposure to hazards,
and
3. while in operation, damaged materials and equipment are examined by a competent person to deter-mine
if they are suitable for continued use. If materials and equipment are not safe for use, they
must be removed from service. These materials cannot be returned to service without the evaluation
and approval of a registered professional engineer.
Sloping and Benching
One method of ensuring the safety and health of workers in a trench or excavation is to slope the sides
of the trench in accordance with OSHA standards. Figure 18 illustrates sloping and benching alterna-tives
that are permitted for certain soil types. The safe slope for the banks of an excavation varies with
different soil types, as discussed earlier, and must be determined on each individual project.
Figure 18
Sloping and Benching
14
20' Maximum
4' Maximum
1
1
Single Bench
This example is allowed only in cohesive Type B soil
Spaced Sheeting
Another popular method of protection is called spaced sheeting. It is also referred to as spot shoring.
This method, shown in figure 19, involves placing spaced timber shores, bracing, trench jacks, piles or
other materials in a manner strong enough to resist the pressures surrounding the excavation. Sheeting
consists of vertical planks used around the boundary of the proposed excavation. Horizontal braces
extend between the vertical planks to support the sheeting. The horizontal trench braces may be wooden
or telescoping metal pipes. The metal braces are typically used when the width of the trench exceeds 5
feet. It is important to remember that all materials selected for use must be in good condition.
Figure 19
Spaced Sheeting
Close Sheeting
This method involves the placement of continuous solid sheeting along the entire length of a trench
excavation. An example is shown in figure 20. The same types of materials used in spaced sheeting can be
used in close sheeting. Timber sheeting or steel sheet piles may be selected for use depending on the cir-cumstances.
As a general rule of thumb, steel sheeting becomes more cost effective when the depth of a
planned excavation exceeds 15 feet. Each of the major components of this system has been labeled in fig-ure
20. Cleats may also be used to fasten the struts to the wales and prevent slipping or falling out.
15
Sheeting
Wales with jack screws
Figure 20
Close Sheeting
Trench Shield
Contractors also may use a trench shield, a prefabricated movable structure often composed of steel
plates welded to a heavy steel frame ( see figure 21). Some trench shields are composed of aluminum or
fiberglass. OSHA standards permit the use of a trench shield as long as the protection it provides is equal
to or greater than the protection that would be provided by the appropriate shoring system. Employees
must know to work only within the protection of the shield. Also, if a slide starts, workers must know
that they should not run out of the shield into the path of the slide.
16
Jack screw Continuous sheet piling
Wale
Metal brace or strut
Figure 21
Trench Shield
Other Systems
Some other systems that are used occasionally include:
• Freezing the moist or saturated soil by the circulation of low- temperature brine through piping dri-ven
into the soil
• Injection of chemical or other grouting into the soil to solidify and fill cracks and space surrounding
the individual soil particles to solidify the soil mass
17
4
Installation and Removal of Protective Systems
To ensure the safety of the workers and the integrity of the job, it is essential to install the various
types of trench protection properly. The standard requires the following procedures for the protection of
employees when installing support systems:
• securely connect members of support systems,
• safely install support systems,
• never overload members of support systems, and
• install other structural members to carry loads imposed on the support system when temporary
removal of individual members is necessary.
In addition, the standard permits excavation of 2 feet ( 0.61 meters) or less below the bottom of the
members of a support or shield system of a trench if:
1. the system is designed to resist the forces calculated for the full depth of the trench, and
2. there are no indications, while the trench is open, of a possible cave- in below the bottom of the sup-port
system.
Also, the installation of support systems must be closely coordinated with the excavation of trenches.
As soon as work is completed, the excavation should be back- filled as the protective system is dismantled.
After the excavation has been cleared, workers should slowly remove the protective system from the bot-tom
up, taking care to release members slowly.
Sloping and Benching
As mentioned earlier, one method of trench protection can be accomplished by sloping the sides of the
trench to the safe angle specified by OSHA excavation standards. The trench is sloped on both sides.
The safe angle to slope the sides of an excavation varies with different kinds of soil and must be deter-mined
on each individual project. When an excavation has a high water table condition, silty material or
loose boulders, or when it is being dug in areas where erosion, deep frost or sliding is apparent, the safe
angle is more gradual ( that is, flatter) ( see figure 22).
Figure 22
Sloping
Problems Associated With Sloping and Benching
1. The spoil accumulated from digging a trench must be placed above and away from the side walls of
the excavation. Otherwise, the weight of the spoil might create an unsafe condition. OSHA requires
that the spoil be kept 2 feet or more from the edge of the excavation or prevented from falling or
rolling into the excavation by the use of retaining devices. This procedure usually requires a wide
storage area.
18
Determine safe slope
from standard
Too steep
2 ft. minimum,
greater distance
is preferred
Spoil
2. Wide excavation areas can expose footings or cause damage to the walls of an adjacent structure
and thereby pose additional hazards to employees.
3. Wide excavation areas can expose or place utilities ( such as electric power, telephone, water, gas,
storm drain or sewer lines) above the angle for a safe slope, causing the unsupported collapse and
failure of the utility line.
4. Wide areas of excavation require the use of large equipment. There may also be hazards in the
movement of the equipment across a larger excavation. For example, excavation under or adjacent
to electric power lines creates a serious hazard to workers and the public.
5. To prevent the collapse of an unsupported bench in an excavation 8 feet or less in depth, the allow-able
height of a bench at the base of an excavation must be 31/ 2 feet or less. The collapse of one bench
can cause a lower bench to fail in a situation where multiple benches have been excavated. For Type
A soil, for example, the standard requires that multiple benches have an overall slope ( from the top
of one side of an excavation to the bottom) of 3/ 4 horizontal to 1 vertical ( see figure 23).
Figure 23
Unsupported Vertically Sided Lower Portion— Maximum Eight Feet in Depth
The contractor should first make a determination of the soil types at the excavation site using the soil
classification system used by OSHA or one of the other acceptable methods described in the standard.
Next, the contractor should consider potential sloping and benching problems, such as those described
above. Finally, after considering all other protection that may be necessary to ensure safe working condi-tions,
the contractor can determine if sloping ( and possibly benching) is the best method to use at that
site. Figure 24 defines the maximum allowable slopes for excavations less than 20 feet deep. This figure is
shown as table B- 1 in the standard.
Figure 24
Maximum Allowable Slopes
19
8' Max
3/ 4
1
31 / 2 ' Max
All excavations more than 8 feet but not more than 12 feet in depth with unsupported
vertically sided lower portions must have a maximum allowable slope of 1: 1 and a
maximum vertical side of 31/ 2 feet.
Soil or rock type Maximum allowable slopes ( H: V) 1
for excavations less than 20 feet deep3
Stable rock Vertical ( 90°)
Type A2 3⁄ 4: 1 ( 53°)
Type B 1: 1 ( 45°)
Type C 11⁄ 2: 1 ( 34°)
Notes:
1 Numbers shown in parentheses next to maximum allowable slopes are angles
expressed in degrees from the horizontal. Angles have been rounded off.
2 A short- term maximum allowable slope of 1⁄ 2H: 1V ( 63°) is allowed in excava-tions
in Type A soil that are 12 feet ( 3.67 m) or less in depth. Short- term maxi-mum
allowable slopes for excavations greater than 12 feet ( 3.67 m) in depth
shall be 3⁄ 4H: 1V ( 53°).
3 Sloping or benching for excavations greater than 20 feet deep shall be
designed by a registered professional engineer.
Benching. There are two basic types of benching: simple and multiple. The type of soil determines the
horizontal to vertical ratio of the benched side. As a general rule, the bottom vertical height of the trench
must not exceed 4 feet ( 1.2 meters) for the first bench. Subsequent benches may be up to a maximum of 5
feet ( 1.5 meters) vertical in Type A soil and 4 feet ( 1.2 meters) in Type B soil to a total trench depth of 20
feet ( 6.0 meters). All subsequent benches must be below the maximum allowable slope for that soil type.
For Type B soil the trench excavation is permitted in cohesive soil only.
Figure 25
Excavation Made in Type B Soil
The OSHA excavations standard provides four options for design and installation of a sloped excava-tion.
See figure 26.
Option 1. Excavations must be sloped at an angle not steeper than 11⁄ 2 horizontal to 1 vertical ( 34
degrees). The exact configuration of the sides must also conform to other requirements ( described in
appendix B of the standard) for sloping of Type C soil.
Option 2. The design and installation may be performed in accordance with the soil classification sys-tem
used by OSHA ( appendix A of the standard), followed by the appropriate sloping requirements listed
in appendix B of the standard.
Option 3. The excavation design and installation of the sloping approach can be performed in accor-dance
with tabulated data ( such as tables and charts) approved for use by a registered professional engi-neer.
Option 4. Sloping and benching systems not utilizing options 1, 2 or 3 ( above) must be approved by a
registered professional engineer.
Additional considerations and other important requirements for proper selection of a sloping system
are contained in the standard.
20
20' Maximum
4' Maximum
1
1
Type B Soil
Single Bench Excavation
( Permitted in cohesive soil only)
Type B Soil
Multiple Bench Excavation
( Permitted in cohesive soil only)
20' Maximum
4' Maximum
4' Maximum
1
1
Figure 26
Slope Configurations: Excavations in Layered Soils
21
20' Maximum
3/ 4
1
Type A Soil
Simple Slope Excavation
20' Maximum
1
1
Type B Soil
Simple Slope Excavation
20' Maximum
11/ 2
1
Type C Soil
Simple Slope Excavation
Type A Soil
over
Type A Type C Soil
Type C
1
1
11/ 2
11/ 2
Type B Soil
over
Type B Type C Soil
Type C
1
1
11/ 2
11/ 2
Type A Soil
over
Type B Soil
Type B
Type A
1
1
1
1
Type C Soil
over
Type C Type B Soil
Type B
1
1
1
11/ 2
Type B Soil
over
Type A Soil
Type B
Type A
1
1
1
3/ 4
Type C Soil
over
Type C Type A Soil
Type A
1
1
11/ 2
3/ 4
In case of an emergency, workers must be able to leave the trench quickly. According to OSHA regula-tions,
when employees are required to be in trenches 4 feet deep or more, adequate means of exit, such as
a ladder or steps, must be provided and located so as to require no more than 25 feet of lateral travel.
Support Systems, Shield Systems and Other Protective Systems
When a trench is excavated, employees who are to work in the excavation must be protected from cave-ins.
In addition to the bracing described earlier, the contractor should consider excavating a wider area
than the minimum necessary. Such additional excavation provides a more comfortable working environ-ment
in the trench. In addition, this extra working area may provide a means for workers to escape unex-pected
crises, such as falling objects or debris. Another common sense strategy for safety in trenches
requires managers to reduce risk by limiting the number of workers in the trench at all times. The only
workers allowed in the trench should be those who are absolutely needed to perform the task at hand.
As the trench is backfilled, the braces and planks can be removed for reuse. If installed and removed prop-erly,
vertical planks and trench braces may be used several times. Spaced sheeting is shown in figure 27.
Figure 27
Spaced Sheeting
Close Sheeting
When the soil is unstable, the excavation should be supported along the entire area of the exposed
trench walls. This can be done by installing continuous sheeting that extends the full depth of the trench,
as depicted in figure 28. Tables have been provided in the standard for the selection of timber sizes for
various excavation depths and widths. The timber sizes listed in the tables are for general reference only
and are not adequate for all soil conditions. Be sure to read the information that explains how to refer-ence
the tables properly.
22
Trench jack
Uprights
Very unstable soil conditions will require the sheeting to be driven prior to digging, with the bracing
installed as excavation proceeds. If steel sheeting is used, a pile driver will be necessary for installation.
For wooden sheeting, a pneumatic hammer is often used. The sheeting should extend one foot above the
surrounding ground to help prevent pipe, tools or equipment from falling into the trench.
Figure 28
Close Sheeting
Trench Shields
OSHA regulations allow the use of portable trench boxes or shields in lieu of fixed shoring systems as
long as an equivalent or greater level of employee protection is provided. In deeper trenches, the trench
shield approach is often the safest. The shields may be made with top and bottom sections for adaptabili-ty
to deep and shallow excavations.
Trench boxes or shields:
may be production type or custom made of steel, aluminum or other equivalent material
must be regularly inspected and properly maintained
must be properly used under the direction of a competent person
Trench boxes are different from shoring because, instead of shoring up or otherwise supporting the
trench face, they are intended primarily to protect workers from cave- ins and similar incidents. The exca-vated
area between the outside of the trench box and the face of the trench should be as small as possible.
The space between the trench boxes and the excavation side are backfilled to prevent lateral movement of
the box. Shields may not be subjected to loads exceeding those which the system was designed to with-stand.
23
Braces
Sheet piles
Cleats
Stringers
Combined use. Trench boxes are generally used in open areas, but they also may be used in combina-tion
with sloping and benching. The box should extend at least 18 inches ( 0.45 meters) above the sur-rounding
area if there is sloping toward excavation. This can be accomplished by providing a benched
area adjacent to the box.
Earth excavation to a depth of 2 feet ( 0.61 meters) below the shield is permitted, but only if the shield
is designed to resist the forces calculated for the full depth of the trench and there are no indications
while the trench is open of possible loss of soil from behind or below the bottom of the support system.
Conditions of this type require observation on the effects of bulging, heaving and boiling as well as sur-charging,
vibration, adjacent structures, etc., on excavating below the bottom of a shield. Careful visual
inspection of the conditions mentioned above is the primary and most prudent approach to hazard identi-fication
and control.
Figure 29
Slope and Shield Configurations
Examples of Other Systems
Many shoring systems are introduced to the marketplace each year. The structural supporting mem-bers
are typically made of wood or metal. For most of these systems, the horizontal trench jacks or braces
are activated using air or hydraulic pumps. These types of systems are illustrated in figure 30.
24
Type A Soil
Supported or shielded
Vertically sided lower portion
Support or shield
system
20' Maximum
1
18" Minimum
3/ 4
Type B Soil
Supported or shielded
Vertically sided lower portion
Support or shield
system
20' Maximum
1
18" Minimum
1
Type C Soil
Supported or shielded
Vertically sided lower portion
Support or shield
system
20' Maximum 1
18" Minimum
11/ 2
Figure 30
Air Shoring or Hydraulic Shoring
Shoring Types
Shoring is the provision of a support system for trench faces used to prevent movement of soil, under-ground
utilities, roadways and foundations. Shoring or shielding is used when the location or depth of the
cut makes sloping back to the maximum allowable slope impractical. Shoring systems consist of posts,
wales, struts and sheeting. There are two basic types of shoring, timber and aluminum hydraulic. See fig-ures
31 and 32.
Hydraulic shoring. The trend today is toward the use of hydraulic shoring, a prefabricated strut
and/ or wale system manufactured of aluminum or steel. Hydraulic shoring provides a critical safety
advantage over timber shoring because workers do not have to enter the trench to install or remove
hydraulic shoring. Other advantages of most hydraulic systems are that they:
• Are light enough to be installed by one worker;
• Are gauge- regulated to ensure even distribution of pressure along the trench line;
• Can have their trench faces “ preloaded” to use the soil’s natural cohesion to prevent movement; and
�� Can be adapted easily to various trench depths and widths.
All shoring should be installed from the top down and removed from the bottom up. Hydraulic shoring
should be checked at least once per shift for leaking hoses and/ or cylinders, broken connections, cracked
nipples, bent bases, and any other damaged or defective parts.
25
18" Maximum
Vertical
spacing
4' Maximum
2' Maximum
Vertical rail
Hydraulic cylinder
Aluminum hydraulic shoring
Examples of trench jacks
used in pneumatic ( air)
and hydraulic shoring
Figure 31
Shoring Variations: Typical Aluminum Hydraulic Shoring Installations
Pneumatic shoring works in a manner similar to hydraulic shoring. The primary difference is that
pneumatic shoring uses air pressure in place of hydraulic pressure. A disadvantage to the use of pneu-matic
shoring is that an air compressor must be on site. Air shoring involves using compressed air
instead of hydraulic fluid to expand the trench jacks into position. Using the air type of system, pins are
put in place to lock the jacks when a desired level of stability has been achieved. For the removal of this
trenching system, air is again injected into the jacks to extend them, allowing the pin to be removed.
These types of jacks are popular since they are cleaner than hydraulic jacks and there is no danger from
the leakage of fluids or other lubrication.
1. Screw jacks. Screw jack systems differ from hydraulic and pneumatic systems in that the struts of a
screw jack system must be adjusted manually. This creates a hazard because the worker is required
to be in the trench in order to adjust the strut. In addition, uniform “ preloading” cannot be achieved
with screw jacks, and their weight creates handling difficulties.
2. Single- cylinder hydraulic shores. Shores of this type are generally used in a water system, as an
assist to timber shoring systems, and in shallow trenches where face stability is required.
26
Vertical Aluminum Hydraulic Shoring
( Spot Bracing)
Vertical Aluminum Hydraulic Shoring
( With Plywood)
Vertical Aluminum Hydraulic Shoring
( Stacked)
Aluminum Hydraulic Shoring Waler System
( Typical)
Vertical
Roll
Vertical
Roll
Vertical
Roll
Plywood
Hydraulic
Cylinder
Hydraulic
Cylinder
Hydraulic
Cylinder
Hydraulic
Cylinder
Upright
Sheeting
Wale
3. Underpinning. This process involves stabilizing adjacent structures, foundations and other intru-sions
that may have an impact on the excavation. As the term indicates, underpinning is a proce-dure
in which the foundation is physically reinforced. Underpinning should be conducted only under
the direction of and with the approval of a registered professional engineer.
Figure 32
Shoring Variations
The excavations standard provides four options for the design of support systems, shield systems and
other protective systems. A summary of the options follows.
Option 1. The soil conditions encountered at the site must first be classified using the soil classification
system used by OSHA ( appendix A of the standard). Based on the soil classification and other project con-ditions,
the contractor may select a timber shoring system using the information contained in appendix C
or an aluminum hydraulic shoring system using the information contained in appendix D. The informa-tion
contained in appendix D should only be used when the manufacturer’s data for an hydraulic system
are not available. The information in appendix D includes tables that detail the maximum vertical and
horizontal spacings that may be used with various aluminum member sizes and various hydraulic cylin-der
sizes.
Option 2. The manufacturer’s tabulated data provided with commercially available support systems,
shield systems or other protective systems may be used if jobsite conditions and methods of use are in
strict accordance with the design intent of the system.
Option 3. Other tabulated data, such as tables and charts, prepared by a registered professional engi-neer
for use under the conditions at the site may be used for the design of support systems, shield sys-tems
and other protective systems.
Option 4. Support systems, shield systems and other protective systems not using options 1, 2 or 3
( above) must be approved by a registered professional engineer.
There is other important information in the standard that must be reviewed to execute one of these
design options correctly.
Other Hazards
Falls and Equipment
In addition to cave- in hazards and secondary hazards related to cave- ins, there are other hazards from
which workers must be protected during excavation- related work. These hazards include exposure to
falls, falling loads and mobile equipment. To protect employees from these hazards, OSHA requires the
employer to take the following precautions:
27
Upright Sheeting
Screw
jack
Wale Pneumatic/ hydraulic jacks
Screw jack
• Keep materials or equipment that might fall or roll into an excavation at least 2 feet ( 0.61 meters)
from the edge of excavations, or have retaining devices, or both.
• Provide warning systems such as mobile equipment, barricades, hand or mechanical signals, or stop
logs to alert operators of the edge of an excavation. If possible, keep the grade away from the excava-tion.
• Provide scaling to remove loose rock or soil or install protective barricades and other equivalent pro-tection
to protect employees against falling rock, soil or materials.
• Prohibit employees from working on faces of sloped or benched excavations at levels above other
employees unless employees at lower levels are adequately protected from the hazard of falling,
rolling or sliding material or equipment.
• Prohibit employees under loads that are handled by lifting or digging equipment. To avoid being
struck by any spillage or falling materials, require employees to stand away from vehicles being
loaded or unloaded. If cabs of vehicles provide adequate protection from falling loads during loading
and unloading operations, the operators may remain in them.
Water Accumulation
The standard prohibits employees from working in excavations where water has accumulated or is
accumulating unless adequate protection has been taken. If water removal equipment is used to control
or prevent water from accumulating, the equipment and operations of the equipment must be monitored
by a competent person to ensure proper use.
OSHA standards also require that diversion ditches, dikes or other suitable means be used to prevent
surface water from entering an excavation and to provide adequate drainage of the area adjacent to the
excavation. Also, a competent person must inspect excavations subject to runoffs from heavy rains.
Hazardous Atmospheres
Under this provision, a competent person must test excavations greater than 4 feet ( 1.22 meters) in
depth as well as ones where oxygen deficiency or a hazardous atmosphere exists or could reasonably be
expected to exist, before an employee enters the excavation. If hazardous conditions exist, controls such
as proper respiratory protection or ventilation must be provided. Also, controls used to reduce atmospher-ic
contaminants to acceptable levels must be tested regularly. Where adverse atmospheric conditions may
exist or develop in an excavation, the employer also must provide and ensure that emergency rescue
equipment, ( e. g., breathing apparatus, a safety harness and line, basket stretcher, etc.) is readily avail-able.
This equipment must be attended when used.
When an employee enters bell- bottom pier holes and similar deep and confined footing excavations, the
employee must wear a harness with a lifeline. The lifeline must be securely attached to the harness and
must be separate from any line used to handle materials. Also, while the employee wearing the lifeline is
in the excavation, an observer must be present to ensure that the lifeline is working properly and to
maintain communication with the employee.
Access and Egress
Under the standard, the employer must provide safe access and egress to all excavations. According to
OSHA regulations, when employees are required to be in trench excavations 4 feet deep ( 1.22 meters) or
more, adequate means of exit, such as ladders, steps, ramps or other safe means of egress, must be pro-vided
and be within 25 feet ( 7.62 meters) of lateral travel. If structural ramps are used as a means of
access or egress, they must be designed by a competent person if used for employee access or egress, or a
competent person qualified in structural design if used by vehicles. Also, structural members used for
ramps or runways must be uniform in thickness and joined in a manner to prevent tripping or displace-ment.
28
5
Residential Contractors and the Excavations Standard
Residential builders and contractors face a unique set of circumstances when building homes. Both the
federal and North Carolina departments of labor have recognized that residential construction sites can
be very different from commercial sites as they relate to part of the OSHA Excavations Standard ( 29 CFR
1926.652). To address accepted residential building practices as they relate to the standard, both agencies
have adopted similar occupational safety and health enforcement policies for excavations on residential
sites.
Residential builders in North Carolina must follow the N. C. Department of Labor’s version of this poli-cy.
The department’s policy suspends 29 CFR 1926.652 for house foundation and basement excavations at
residential sites when all of the following conditions are present:
• The house foundation/ basement excavation is less than 71/ 2 feet in depth or is benched for at least 2
feet horizontally for every 5 feet or less of vertical height.
• The minimum horizontal width ( excavation face to formwork/ wall) at the bottom of the excavation is
as wide as practicable but not less than 2 feet.
• There is no water, surface tension cracks or other environmental conditions present that reduce the
stability of the excavations.
• There is no heavy equipment operating in the vicinity that causes vibration to the excavation while
employees are in the excavation. All soil, equipment and material surcharge loads are no closer in
distance to the top edge of the excavation than the excavation is deep; however, when front- end load-ers
are used to dig the excavations, the soil surcharge load must be placed as far back from the edge
of the excavation as possible, but never closer than 2 feet.
• Work crews in the excavation are the minimum number needed to perform the work.
• The work has been planned and is carried out in a manner to minimize the time employees are in
the excavation.
29
6
Worker Training and Jobsite Safety
Suggested Company Policy
One of the most important responsibilities of field and office management is planning for safety. Most
on- the- job problems and accidents directly result from improper planning. Correcting mistakes in shoring
and/ or sloping after work has begun slows down the operation, adds to the cost and increases the possibil-ity
of an excavation failure.
Contractors should develop safety checklists to make certain that there is enough information about
the jobsite and that all needed items, such as safety equipment, are on hand.
To help ensure safety in trenching and excavations, these specific conditions should be taken into
account:
• Soil types and layers
• Traffic
• Nearness of structures and their condition
• Surface and ground water conditions
• The water table elevation
• Overhead and underground utilities
• Weather
These and other conditions can be determined by jobsite studies, observations, test borings and consul-tations
with local officials and utility companies. Underground installations— sewer, telephone, water,
fuel and electric lines— that may be encountered in the excavation must be located before starting the job.
If underground installations are uncovered, OSHA regulations require that they be properly supported.
The contractor must contact the utility companies involved and inform them of the proposed work before
starting the trench or excavation.
Companies should establish a safety and health program. This important aspect of the work is put into
place before the job starts. Field and office personnel should become familiar with the company policies
and guidelines outlined in their company safety program. The program is often put in writing to solidify
and communicate the company’s position with regard to jobsite safety. Additionally, OSHA has specific
training requirements for all employees who are required to enter confined or enclosed spaces.
Cooperation from supervisors, employee groups and individual employees is necessary to ensure that
safety policies are implemented effectively. In addition, each supervisor must understand his or her
degree of responsibility for providing a safe working environment.
The cooperation of all employees requires their recognition of safety hazards and the necessary safety
precautions. Employees should be trained in the following areas:
1. Hazards associated with trenching and excavating
2. Soil identification
3. Safe slopes for different soil types and conditions
4. Proper installation and shoring
5. Stress patterns on trench walls from soil and spoil, equipment, and vibration caused by equipment
and traffic
6. Effects of adjacent buried utilities, building foundations and lengthy exposure to the elements on
trench side walls and other excavations
30
7. Effects on trench and excavation conditions from severe weather, such as excess water, freezing tem-peratures,
unexpected heat or prolonged drying
8. Recognition of buried drums, containers, tanks and wells
Employees should be trained to follow proper procedures to involve the electrical power company,
health department, and other agencies upon the discovery of unforeseen objects such as wells, sewage
disposal systems, cemeteries, and historic or architectural artifacts.
An example of a company safety and health program in action is a written policy that ensures that all
employees in all excavations will be protected from cave- ins. All company personnel will be expected to do
their part to enforce this policy.
On- the- Job Follow- Up
Once the job gets underway, each employer should keep itself informed of the safety aspects of the pro-ject
as well as the progress of the work. This is called on- the- job follow- up and involves a series of inspec-tions
to detect hazards and correct jobsite situations before cave- ins or other accidents occur. When man-agement
requires daily reports, acts on the reports and makes personal visits to the jobsite, it may feel
more confident that everyone is meeting job safety responsibilities.
Special Health and Safety Considerations
Competent person. The designated competent person should have and be able to demonstrate the fol-lowing:
Training, experience and knowledge of:
- soil analysis;
- use of protective systems; and
- requirements of 29 CFR Part 1926 Subpart P.
Ability to detect:
- conditions that could result in cave- ins;
- failures in protective systems;
- hazardous atmospheres; and
- other hazards including those associated with confined spaces.
Authority to take prompt corrective measures to eliminate existing and predictable hazards and to stop
work when required.
Surface crossing of trenches. Surface crossing of trenches should be discouraged; however, if
trenches must be crossed, such crossings are permitted only under the following conditions:
Vehicle crossings must be designed by and installed under the supervision of a registered professional
engineer.
Walkways or bridges must be provided for foot traffic. These structures must:
- have a safety factor of 4;
- have a minimum clear width of 20 inches ( 0.51 meters);
- be fitted with standard rails; and
- extend a minimum of 24 inches ( 0.61 meters) past the surface edge of the trench.
Ingress and egress. Access to and exit from the trench require the following conditions:
Trenches 4 feet or more in depth should be provided with a fixed means of egress. Spacing between lad-ders
or other means of egress must be such that a worker will not have to travel more than 25 feet lat-erally
to the nearest means of egress. Ladders must be secured and extend a minimum of 36 inches ( 0.9
meters) above the landing. Metal ladders should be used with caution, particularly when electric utili-ties
are present.
31
Exposure to vehicles. Procedures to protect employees from being injured or killed by vehicle traffic
include:
Providing employees with and requiring them to wear warning vests or other suitable garments
marked with or made of reflectorized or high- visibility materials. Requiring a designated, trained flag-person
along with signs, signals and barricades when necessary.
Exposure to falling loads. Employees must be protected from loads or objects falling from lifting or
digging equipment. Procedures designed to ensure their protection include:
Employees are not permitted to work under raised loads. Employees are required to stand away from
equipment that is being loaded or unloaded. Equipment operators or truck drivers may stay in their
equipment during loading and unloading if the equipment is properly equipped with a cab shield or
adequate canopy.
Warning systems for mobile equipment. The following steps should be taken to prevent vehicles
from accidentally falling into the trench:
Barricades must be installed where necessary. Hand or mechanical signals must be used as required.
Stop logs must be installed if there is a danger of vehicles falling into the trench. Soil should be graded
away from the excavation; this will assist in vehicle control and channeling of run- off water.
Hazardous atmospheres and confined spaces. Employees must not be permitted to work in haz-ardous
and/ or toxic atmospheres. Such atmospheres include those with:
Less than 19.5 percent or more than 23.5 percent oxygen; combustible gas concentration greater than
20 percent of the lower flammable limit; and concentrations of hazardous substances that exceed those
specified in the Threshold Limit Values for Airborne Contaminants established by the American
Conference of Governmental Industrial Hygienists.
All operations involving such atmospheres must be conducted in accordance with OSHA requirements
for occupational health and environmental controls ( see Subpart D of 29 CPR 1926) for personal protec-tive
equipment and for lifesaving equipment ( see Subpart E, 29 CFR 1926). Engineering controls ( e. g.,
ventilation) and respiratory protection may be required.
When testing for atmospheric contaminants, the following should be considered:
• Testing should be conducted before employees enter the trench and should be done regularly to
ensure that the trench remains safe. The frequency of testing should be increased if equipment is
operating in the trench. Testing frequency should also be increased if welding, cutting or burning is
done in the trench.
• Employees required to wear respiratory protection must be trained, fit- tested and enrolled in a res-piratory
protection program. Some trenches qualify as confined spaces. When this occurs, compli-ance
with the Confined Space Standard is also required.
Emergency rescue equipment. Emergency rescue equipment is required when a hazardous atmos-phere
exists or can reasonably be expected to exist. Requirements are as follows:
• Respirators must be of the type suitable for the exposure. Employees must be trained in their use,
and a respirator program must be instituted.
• Attended ( at all times) lifelines must be provided when employees enter bell- bottom pier holes, deep
confined spaces or other similar hazards.
• Employees who enter confined spaces must be trained.
Standing water and water accumulation. Methods for controlling standing water and water accu-mulation
must be provided and should consist of the following if employees are permitted to work in the
excavation:
• Use of special support or shield systems approved by a registered professional engineer
• Water removal equipment, i. e., well pointing, used and monitored by a competent person
32
• Safety harnesses and lifelines used in conformance with 29 CFR 1926.104
• Surface water diverted away from the trench
• Employees removed from the trench during rainstorms
• Trenches carefully inspected by a competent person after each rain and before employees are permit-ted
to re- enter the trench.
Inspections. A competent person must inspect the trench and document any findings. The following
guide specifies the frequency and conditions requiring inspections:
• Daily and before the start of each shift
• As dictated by the work being done in the trench
• After every rainstorm
• After other events that could increase hazards, e. g. snowstorm, windstorm, thaw, earthquake, etc.
• When fissures, tension cracks, sloughing, undercutting, water seepage, bulging at the bottom or
other similar conditions occur
• When there is a change in the size, location or placement of the spoil pile
• When there is any indication of change or movement in adjacent structures.
33
7
Safety Checklist*
FOR A SAFE JOBSITE TRENCHING OPERATION, THE ANSWER TO EACH OF THE FOLLOW-ING
SHOULD BE YES.
YES NO
General Protection Requirements
____ ____ Are all walkways, runways and sidewalks clear of excavated material or other obstructions?
____ ____ Are all undermined sidewalks shored to carry a minimum live load of 125 pounds per square
foot?
____ ____ Are planks laid parallel to the length of walk and fastened together against displacement if
used for raised walkways, runways or sidewalks?
____ ____ Are planks uniform in thickness?
____ ____ Are cleats on ramps provided when ramps are used in lieu of steps?
____ ____ Are all employees engaged in excavation, trenching and shoring provided with the appropriate
training and personal protective equipment?
____ ____ Are employees exposed to vehicular traffic wearing warning vests marked with or made of
reflectorized or high visibility material?
____ ____ Are employees prohibited from being under suspended loads handled by lifting and other
heavy equipment?
____ ____ Are daily inspections made of excavations, trenching and shoring?
____ ____ If evidence of possible cave- ins or slides is apparent, does all work stop until the necessary
precautions have been taken to safeguard all the employees?
Specific Excavation Requirements
____ ____ Prior to opening an excavation and/ or trench, has an effort been made to determine if any
underground installations will be encountered?
____ ____ Did you contact utility companies before excavating?
____ ____ Did you remove all trees, boulders and other surface encumbrances before excavating was
begun?
____ ____ Are the walls and faces of all excavations in which employees are exposed to danger from mov-ing
ground guarded by a shoring system, ground sloping or some other equivalent means?
____ ____ Are excavations inspected by a competent person after every rainstorm or other hazard-increasing
occurrence to determine if additional protection against slides and cave- ins is nec-essary?
____ ____ Did you store or retain excavated or other materials at least 2 feet or more from the edges of
the excavation?
____ ____ Do sides, slopes and faces of all excavations meet accepted engineering requirements by tier-ing,
barricading, rock bolting, wire meshing or other equally effective means?
____ ____ When excavations are in excess of 20 feet in depth, was the support system designed by a reg-istered
professional engineer?
34
* Adapted from Checklist for Trenching and Shoring, Kentucky Occupational Safety and Health Administration
( please see Suggested Readings).
YES NO
____ ____ Are all materials used for sheeting, piling, cribbing, bracing, shoring and underpinning in
good serviceable condition?
____ ____ Is the timber used for this purpose free of large or loose knots?
____ ____ Are diversion ditches, dikes or other suitable means used to prevent surface water from enter-ing
an excavation and to provide adequate drainage to the area?
____ ____ When heavy equipment, material or objects are on a level above and near an excavation, are
the sides of the excavation sheet- piled, shored and braced?
____ ____ When mobile equipment is used or is adjacent to excavations, are stop logs or barricades
installed?
____ ____ Are adequate barriers provided at all remotely located excavations? Are all wells, pits, shafts,
etc., adequately covered?
____ ____ On completion of the operation, are all temporary wells, pits, shafts, etc., backfilled?
____ ____ If possible, are dust conditions kept to a minimum?
____ ____ In locations where oxygen deficiency or gaseous conditions are possible, is the air in the exca-vation
tested?
____ ____ Where employees and/ or equipment are required to cross over excavations, are walkways or
bridges with standard guardrails provided?
____ ____ Do ladders used in excavation operations meet the OSHA requirements, and are ladders suffi-cient
in number?
____ ____ Are ladders or other means of exiting the trench provided and located so as to require no more
than 25 feet of lateral travel by employees to use them?
Specific Trenching Requirements
____ ____ Are all banks more than 5 feet high shored or laid back to a stable slope?
____ ____ Are the sides of trenches or embankments in Type B soil 4 feet or more in depth shored, sheet-ed,
braced, benched, sloped or otherwise sufficiently supported?
____ ____ Are sides of trenches or embankments in Type A soil shored or otherwise supported when the
excavation is 4 feet deep?
____ ____ When excavations or trenches are located near backfills or where they are subjected to vibra-tions
from railroad or highway traffic or machinery operation, are precautions such as shoring
and bracing taken to prevent slides or cave- ins?
____ ____ Is the excavation braced and shored as soon as it is opened?
____ ____ Are all crossbraces or jacks placed in true horizontal position?
____ ____ Are the vertical braces properly spaced and secured to prevent sliding, falling or kickouts?
____ ____ Where trench boxes or shields are used, are they designed and constructed in a manner that
will provide protection for the employees?
____ ____ Do backfilling and removal of trench supports progress together from the bottom of the
trench?
____ ____ Are jacks or braces released slowly?
____ ____ In unstable soil, are ropes used to pull out jacks or braces from above, after employees are
safely away from the trench?
35
Definitions
Accepted engineering practices are procedures compatible with the standards of practice required
of a registered professional engineer.
Adjacent structure stability refers to the stability of the foundations of adjacent structures whose
location may create surcharges, changes in soil conditions or other disruptions that have the potential to
extend into the failure zone of the excavation or trench.
Competent person is an individual who is capable of identifying existing and predictable hazards or
working conditions that are hazardous, unsanitary or dangerous to employees, and who has authoriza-tion
to take prompt corrective measures to eliminate or control these hazards and conditions.
Confined space is a space that, by design and/ or configuration, has limited openings for entry and
exit, unfavorable natural ventilation, may contain or produce hazardous substances, and is not intended
for continuous employee occupancy.
Excavation. An excavation is any man- made cut, cavity, trench or depression in an earth surface that
is formed by earth removal. A trench is a narrow excavation ( in relation to its length) made below the
surface of the ground. In general, the depth of a trench is greater than its width, and the width ( mea-sured
at the bottom) is not greater than 15 feet ( 4.6 meters). If a form or other structure installed or con-structed
in an excavation reduces the distance between the form and the side of the excavation to 15 feet
( 4.6 meters) or less ( measured at the bottom of the excavation), the excavation is also considered to be a
trench.
Hazardous atmosphere is an atmosphere that by reason of being explosive, flammable, poisonous,
corrosive, oxidizing, irritating, oxygen- deficient, toxic or otherwise harmful may cause death, illness or
injury to persons exposed to it.
Ingress and egress mean “ entry” and “ exit,” respectively. In trenching and excavation operations,
they refer to the provision of safe means for employees to enter or exit an excavation or trench.
Protective system refers to a method of protecting employees from cave- ins, from material that could
fall or roll from an excavation face or into an excavation, and from the collapse of adjacent structures.
Protective systems include support systems, sloping and benching systems, shield systems, and other sys-tems
that provide the necessary protection.
Registered professional engineer is a person who is registered as a professional engineer in the
state where the work is to be performed. However, a professional engineer who is registered in any state
is deemed to be a registered professional engineer within the meaning of Subpart P when approving
designs for manufactured protective systems or tabulated data to be used in interstate commerce.
Support system refers to structures such as underpinning, bracing and shoring that provide support
to an adjacent structure or underground installation or to the sides of an excavation or trench.
Subsurface encumbrances include underground utilities, foundations, streams, water tables, trans-former
vaults and geological anomalies.
Surcharge means an excessive vertical load or weight caused by spoil, overburden, vehicles, equip-ment
or activities that may affect trench stability.
Tabulated data are tables and charts approved by a registered professional engineer and used to
design and construct a protective system.
Underground installations include, but are not limited to, utilities ( sewer, telephone, fuel, electric,
water and other product lines), tunnels, shafts, vaults, foundations and other underground fixtures or
equipment that may be encountered during excavation or trenching work.
Unconfined compressive strength is the load per unit area at which soil will fail in compression.
This measure can be determined by laboratory testing, or it can be estimated in the field using a pocket
penetrometer, by thumb penetration tests or by other methods.
36
Suggested Readings
General References
29 CFR 1926 § § 650– 652 and appendixes A– E
Brunst, Gregory J. “ Unique Shoring Equipment Increase Trench Safety.” Concrete Construction ( June
1986): 46.
Carson, Brinton A. General Excavation Methods. New York: F. W. Dodge Corp., 1961.
Checklist for Trenching and Shoring ( Revised). Kentucky Labor Cabinet, Occupational Safety and Health
Program, Division of Education and Training. April 1985.
“ Cut and Cover Construction on Unstable Slopes.” Journal of the Construction Division. Proceeding of
ASCE. Vol. 106, No. C04 ( December 1980): 585– 597.
Excavations. Occupational Safety and Health Administration. OSHA 2226.
Manual of Accident Prevention in Construction. Washington, D. C.: Associated General Contractors of
America, 1971.
Safety and Health in Excavation and Trenching Operations. U. S. Department of Labor, Occupational
Safety and Health Administration. 2091.
“ Trenching Safety for Concrete Contractors.” Concrete Construction ( June 1986): 556– 559.
Technical References
American Standard Safety Code for Building Construction. Institute of Architects. National Safety
Council. June 1944.
Durham, C. W. Foundation of Structures. New York: McGraw- Hill Book Company Inc., 1950.
“ Excavation Stabilized Using Rock Revetments.” Journal of Geotechnical Engineering. Vol. 109, No. 3
( March 1983): 424– 439.
“ Field Measurements of an Earth Support System.” Journal of the Geotechnical Engineering Division.
Proceeding of ASCE. Vol. 107, No. GT 12 ( December 1981): 1625– 1642.
Keen, B. The Physical Properties of Soil. New York: Longmans, Green, and Company Inc., 1931.
Krynine, D. P., and W. R. Judd. Principles of Engineering Geology and Geotechnics. New York: McGraw-
Hill Book Company Inc., 1957.
Nunnally, S. W. Construction Methods and Management. New Jersey: Simon and Schuster, 1987.
Peck, Hanson and Thornburn. Foundation Engineering. New York: John Wiley and Sons, 1974.
Peck, R. B., et al. Foundation Engineering. New York: John Wiley and Sons, 1974.
Peurifoy, Robert L., and Garold D. Oberlender. Estimating Construction Costs. New York: McGraw- Hill
Book Company Inc., 1989. 117– 122.
Swiger, F. W. “ Control of Ground Water in Excavations.” ASCE Journal Construction Division. Vol. 86, No.
C01: 41– 53.
Terzaghi, K., and R. B. Peck. Soil Mechanics in Engineering Practice. New York: John Wiley and Sons,
1948.
“ Trench Excavation Effects on Adjacent Buried Pipes: Finite Element Study.” Journal of Geotechnical
Engineering. Vol. 109, No. 11 ( November 1983): 1399– 1415.
U. S. Department of Labor, OSHA Technical Manual ( Section V: Chapter II), Excavations: Hazard
Recognition in Trenching and Shoring.
37
Werblin, D. A. “ Installation and Operation of Dewatering Systems.” ASCE Journal Soil Mech. 86 ( 1960):
47– 66.
Yokel, Tucker, Lymon, Reese. Soil Classification for Construction Practice in Shallow Trenching. U. S.
Department of Commerce, National Bureau of Standards.
38
The following industry guides are available from the N. C. Department of Labor’s Division of Occupational
Safety and Health:
1# 1. A Guide to Safety in Confined Spaces
1# 2. A Guide to Procedures of the Safety and Health Review Board of North Carolina
1# 3. A Guide to Machine Safeguarding
1# 4. A Guide to OSHA in North Carolina
1# 5. A Guide for Persons Employed in Cotton Dust Environments
1# 6. A Guide to Lead Exposure in the Construction Industry
1# 7. A Guide to Bloodborne Pathogens in the Workplace
1# 8. A Guide to Voluntary Training and Training Requirements in OSHA Standards
1# 9. A Guide to Ergonomics
# 10. A Guide to Farm Safety and Health
# 11. A Guide to Radio Frequency Hazards With Electric Detonators
# 12. A Guide to Forklift Operator Training
# 13. A Guide to the Safe Storage of Explosive Materials
# 14. A Guide to the OSHA Excavations Standard
# 15. A Guide to Developing and Maintaining an Effective Hearing Conservation Program
# 17. A Guide to Asbestos for Industry
# 18. A Guide to Electrical Safety
# 19. A Guide to Occupational Exposure to Wood, Wood Dust and Combustible Dust Hazards
# 20. A Guide to Crane Safety
# 21. A Guide to School Safety and Health
# 23. A Guide to Working With Electricity
# 25. A Guide to Personal Protective Equipment
# 26. A Guide to Manual Materials Handling and Back Safety
# 27. A Guide to the Control of Hazardous Energy ( Lockout/ Tagout)
# 28. A Guide to Eye Wash and Safety Shower Facilities
# 29. A Guide to Safety and Health in Feed and Grain Mills
# 30. A Guide to Working With Corrosive Substances
# 31. A Guide to Formaldehyde
# 32. A Guide to Fall Prevention in Industry
# 33. A Guide to Office Safety and Health
# 34. A Guide to Safety and Health in the Poultry Industry
# 35. A Guide to Preventing Heat Stress
# 36. A Guide to the Safe Use of Escalators and Elevators
# 37. A Guide to Boilers and Pressure Vessels
# 38. A Guide to Safe Scaffolding
# 39. A Guide to Safety in the Textile Industry
# 40. A Guide to Emergency Action Planning
# 41. A Guide to OSHA for Small Businesses in North Carolina
Occupational Safety and Health ( OSH)
Sources of Information
You may call 1- 800- NC- LABOR ( 1- 800- 625- 2267) to reach any division of the N. C. Department of Labor; or visit the
NCDOL home page on the World Wide Web, Internet Web site address: http:// www. nclabor. com.
N. C. Division of Occupational Safety and Health
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 3rd Floor)
Local Telephone: ( 919) 807- 2900 Fax: ( 919) 807- 2856
For information concerning education, training and interpretations of occupational safety and health standards contact:
Bureau of Education, Training and Technical Assistance
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 4th Floor)
Telephone: ( 919) 807- 2875 Fax: ( 919) 807- 2876
For information concerning occupational safety and health consultative services and safety awards programs contact:
Bureau of Consultative Services
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 3rd Floor)
Telephone: ( 919) 807- 2899 Fax: ( 919) 807- 2902
For information concerning migrant housing inspections and other related activities contact:
Agricultural Safety and Health Bureau
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 2nd Floor)
Telephone: ( 919) 807- 2923 Fax: ( 919) 807- 2924
For information concerning occupational safety and health compliance contact:
Safety and Health Compliance District Offices
Raleigh District Office ( 313 Chapanoke Road, Raleigh, NC 27603)
Telephone: ( 919) 779- 8570 Fax: ( 919) 662- 4709
Asheville District Office ( 204 Charlotte Highway, Suite B, Asheville, NC 28803- 8681)
Telephone: ( 828) 299- 8232 Fax: ( 828) 299- 8266
Charlotte District Office ( 901 Blairhill Road, Suite 200, Charlotte, NC 28217- 1578)
Telephone: ( 704) 665- 4341 Fax: ( 704) 665- 4342
Winston- Salem District Office ( 4964 University Parkway, Suite 202, Winston- Salem, NC 27106- 2800)
Telephone: ( 336) 776- 4420 Fax: ( 336) 776- 4422
Wilmington District Office ( 1200 N. 23rd St., Suite 205, Wilmington, NC 28405- 1824)
Telephone: ( 910) 251- 2678 Fax: ( 910) 251- 2654
*** To make an OSHA Complaint, OSH Complaint Desk: ( 919) 807- 2796***
For statistical information concerning program activities contact:
Planning, Statistics and Information Management
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 2nd Floor)
Telephone: ( 919) 807- 2950 Fax: ( 919) 807- 2951
For information about books, periodicals, vertical files, videos, films, audio/ slide sets and computer databases contact:
N. C. Department of Labor Library
Mailing Address: Physical Location:
1101 Mail Service Center 111 Hillsborough St.
Raleigh, NC 27699- 1101 ( Old Revenue Building, 5th Floor)
Telephone: ( 919) 807- 2848 Fax: ( 919) 807- 2849
N. C. Department of Labor ( Other than OSH)
1101 Mail Service Center
Raleigh, NC 27699- 1101
Telephone: ( 919) 733- 7166 Fax: ( 919) 733- 6197